Truth table

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Truth_table

A truth table is a mathematical table used in logic—specifically in connection with Boolean algebra, boolean functions, and propositional calculus—which sets out the functional values of logical expressions on each of their functional arguments, that is, for each combination of values taken by their logical variables. In particular, truth tables can be used to show whether a propositional expression is true for all legitimate input values, that is, logically valid.

A truth table has one column for each input variable (for example, A and B), and one final column showing all of the possible results of the logical operation that the table represents (for example, A XOR B). Each row of the truth table contains one possible configuration of the input variables (for instance, A=true, B=false), and the result of the operation for those values.

A truth table is a structured representation that presents all possible combinations of truth values for the input variables of a Boolean function and their corresponding output values. A function f from A to F is a special relation, a subset of A×F, which simply means that f can be listed as a list of input-output pairs. Clearly, for the Boolean functions, the output belongs to a binary set, i.e. F = {0, 1}. For an n-ary Boolean function, the inputs come from a domain that is itself a Cartesian product of binary sets corresponding to the input Boolean variables. For example for a binary function, f(A, B), the domain of f is A×B, which can be listed as: A×B = {(A = 0, B = 0), (A = 0, B = 1), (A = 1, B = 0), (A = 1, B = 1)}. Each element in the domain represents a combination of input values for the variables A and B. These combinations now can be combined with the output of the function corresponding to that combination, thus forming the set of input-output pairs as a special relation that is subset of A×F. For a relation to be a function, the special requirement is that each element of the domain of the function must be mapped to one and only one member of the codomain. Thus, the function f itself can be listed as: f = {((0, 0), f₀), ((0, 1), f₁), ((1, 0), f₂), ((1, 1), f₃)}, where f₀, f₁, f₂, and f₃ are each Boolean, 0 or 1, values as members of the codomain {0, 1}, as the outputs corresponding to the member of the domain, respectively. Rather than a list (set) given above, the truth table then presents these input-output pairs in a tabular format, in which each row corresponds to a member of the domain paired with its corresponding output value, 0 or 1. Of course, for the Boolean functions, we do not have to list all the members of the domain with their images in the codomain; we can simply list the mappings that map the member to "1", because all the others will have to be mapped to "0" automatically (that leads us to the minterms idea).

Ludwig Wittgenstein is generally credited with inventing and popularizing the truth table in his Tractatus Logico-Philosophicus, which was completed in 1918 and published in 1921. Such a system was also independently proposed in 1921 by Emil Leon Post.

Nullary operations

There are 2 nullary operations:

• Always true
• Never true, unary falsum

Logical true

The output value is always true, because this operator has zero operands and therefore no input values

p	T
T	T
F	T

Logical false

The output value is never true: that is, always false, because this operator has zero operands and therefore no input values

p	F
T	F
F	F

Unary operations

There are 2 unary operations:

• Unary identity
• Unary negation

Logical identity

Logical identity is an operation on one logical value p, for which the output value remains p.

The truth table for the logical identity operator is as follows:

p	p
T	T
F	F

Logical negation

Logical negation is an operation on one logical value, typically the value of a proposition, that produces a value of true if its operand is false and a value of false if its operand is true.

The truth table for NOT p (also written as ¬p, Np, Fpq, or ~p) is as follows:

p	¬p
T	F
F	T

Binary operations

There are 16 possible truth functions of two binary variables, each operator has its own name.

Truth table

Here is an extended truth table giving definitions of all sixteen possible truth functions of two Boolean variables p and q:

p	q		F0	NOR1	↚2	¬p3	NIMPLY4	¬q5	XOR6	NAND7	AND8	XNOR9	q10	IMPLY11	p12	←13	OR14	T15
T	T		F	F	F	F	F	F	F	F	T	T	T	T	T	T	T	T
T	F		F	F	F	F	T	T	T	T	F	F	F	F	T	T	T	T
F	T		F	F	T	T	F	F	T	T	F	F	T	T	F	F	T	T
F	F		F	T	F	T	F	T	F	T	F	T	F	T	F	T	F	T
Com			✓	✓					✓	✓	✓	✓					✓	✓
Assoc			✓						✓		✓	✓	✓		✓		✓	✓
Adj			F0	NOR1	↛4	¬q5	NIMPLY2	¬p3	XOR6	NAND7	AND8	XNOR9	p12	IMPLY13	q10	→11	OR14	T15
Neg			T15	OR14	←13	p12	IMPLY11	q10	XNOR9	AND8	NAND7	XOR6	¬q5	NIMPLY4	¬p3	↚2	NOR1	F0
Dual			T15	NAND7	→11	¬p3	←13	¬q5	XNOR9	NOR1	OR14	XOR6	q10	↚2	p12	↛4	AND8	F0
L id					F				F		T	T	T,F	T			F
R id							F		F		T	T			T,F	T	F

where

T = true.

F = false.

The superscripts ⁰ to ¹⁵ is the number resulting from reading the four truth values as a binary number with F = 0 and T = 1.

The Com row indicates whether an operator, op, is commutative - P op Q = Q op P.

The Assoc row indicates whether an operator, op, is associative - (P op Q) op R = P op (Q op R).

The Adj row shows the operator op2 such that P op Q = Q op2 P.

The Neg row shows the operator op2 such that P op Q = ¬(P op2 Q).

The Dual row shows the dual operation obtained by interchanging T with F, and AND with OR.

The L id row shows the operator's left identities if it has any - values I such that I op Q = Q.

The R id row shows the operator's right identities if it has any - values I such that P op I = P.

Wittgenstein table

In proposition 5.101 of the Tractatus Logico-Philosophicus, Wittgenstein listed the table above as follows:

	Truthvalues		Operator		Operation name	Tractatus
0	(F F F F)(p, q)	⊥	false	Opq	Contradiction	if p then p; and if q then q
1	(F F F T)(p, q)	NOR	p ↓ q	Xpq	Logical NOR	neither p nor q
2	(F F T F)(p, q)	↚	p ↚ q	Mpq	Converse nonimplication	q and not p
3	(F F T T)(p, q)	¬p, ~p	¬p	Np, Fpq	Negation	not p
4	(F T F F)(p, q)	↛	p ↛ q	Lpq	Material nonimplication	p and not q
5	(F T F T)(p, q)	¬q, ~q	¬q	Nq, Gpq	Negation	not q
6	(F T T F)(p, q)	XOR	p ⊕ q	Jpq	Exclusive disjunction	p or q, but not both
7	(F T T T)(p, q)	NAND	p ↑ q	Dpq	Logical NAND	not both p and q
8	(T F F F)(p, q)	AND	p ∧ q	Kpq	Logical conjunction	p and q
9	(T F F T)(p, q)	XNOR	p iff q	Epq	Logical biconditional	if p then q; and if q then p
10	(T F T F)(p, q)	q	q	Hpq	Projection function	q
11	(T F T T)(p, q)	p → q	if p then q	Cpq	Material implication	if p then q
12	(T T F F)(p, q)	p	p	Ipq	Projection function	p
13	(T T F T)(p, q)	p ← q	if q then p	Bpq	Converse implication	if q then p
14	(T T T F)(p, q)	OR	p ∨ q	Apq	Logical disjunction	p or q
15	(T T T T)(p, q)	⊤	true	Vpq	Tautology	p and not p; and q and not q

The truth table represented by each row is obtained by appending the sequence given in Truthvalues_row to the table

p	T	T	F	F
q	T	F	T	F

For example, the table

p	T	T	F	F
q	T	F	T	F
11	T	F	T	T

represents the truth table for Material implication.

Logical operators can also be visualized using Venn diagrams.

Logical conjunction (AND)

Logical conjunction is an operation on two logical values, typically the values of two propositions, that produces a value of true if both of its operands are true.

The truth table for p AND q (also written as p ∧ q, Kpq, p & q, or p ${\displaystyle \cdot }$ q) is as follows:

p	q	p ∧ q
T	T	T
T	F	F
F	T	F
F	F	F

In ordinary language terms, if both p and q are true, then the conjunction p ∧ q is true. For all other assignments of logical values to p and to q the conjunction p ∧ q is false.

It can also be said that if p, then p ∧ q is q, otherwise p ∧ q is p.

Logical disjunction (OR)

Logical disjunction is an operation on two logical values, typically the values of two propositions, that produces a value of true if at least one of its operands is true.

The truth table for p OR q (also written as p ∨ q, Apq, p || q, or p + q) is as follows:

p	q	p ∨ q
T	T	T
T	F	T
F	T	T
F	F	F

Stated in English, if p, then p ∨ q is p, otherwise p ∨ q is q.

Logical implication

Logical implication and the material conditional are both associated with an operation on two logical values, typically the values of two propositions, which produces a value of false if the first operand is true and the second operand is false, and a value of true otherwise.

The truth table associated with the logical implication p implies q (symbolized as p ⇒ q, or more rarely Cpq) is as follows:

p	q	p ⇒ q
T	T	T
T	F	F
F	T	T
F	F	T

The truth table associated with the material conditional if p then q (symbolized as p → q) is as follows:

p	q	p → q
T	T	T
T	F	F
F	T	T
F	F	T

It may also be useful to note that p ⇒ q and p → q are equivalent to ¬p ∨ q.

Logical equality

Logical equality (also known as biconditional or exclusive nor) is an operation on two logical values, typically the values of two propositions, that produces a value of true if both operands are false or both operands are true.

The truth table for p XNOR q (also written as p ↔ q, Epq, p = q, or p ≡ q) is as follows:

p	q	p ↔ q
T	T	T
T	F	F
F	T	F
F	F	T

So p EQ q is true if p and q have the same truth value (both true or both false), and false if they have different truth values.

Exclusive disjunction

Exclusive disjunction is an operation on two logical values, typically the values of two propositions, that produces a value of true if one but not both of its operands is true.

The truth table for p XOR q (also written as Jpq, or p ⊕ q) is as follows:

p	q	p ⊕ q
T	T	F
T	F	T
F	T	T
F	F	F

For two propositions, XOR can also be written as (p ∧ ¬q) ∨ (¬p ∧ q).

Logical NAND

The logical NAND is an operation on two logical values, typically the values of two propositions, that produces a value of false if both of its operands are true. In other words, it produces a value of true if at least one of its operands is false.

The truth table for p NAND q (also written as p ↑ q, Dpq, or p | q) is as follows:

p	q	p ↑ q
T	T	F
T	F	T
F	T	T
F	F	T

It is frequently useful to express a logical operation as a compound operation, that is, as an operation that is built up or composed from other operations. Many such compositions are possible, depending on the operations that are taken as basic or "primitive" and the operations that are taken as composite or "derivative".

In the case of logical NAND, it is clearly expressible as a compound of NOT and AND.

The negation of a conjunction: ¬(p ∧ q), and the disjunction of negations: (¬p) ∨ (¬q) can be tabulated as follows:

p	q	p ∧ q	¬(p ∧ q)	¬p	¬q	(¬p) ∨ (¬q)
T	T	T	F	F	F	F
T	F	F	T	F	T	T
F	T	F	T	T	F	T
F	F	F	T	T	T	T

Logical NOR

The logical NOR is an operation on two logical values, typically the values of two propositions, that produces a value of true if both of its operands are false. In other words, it produces a value of false if at least one of its operands is true. ↓ is also known as the Peirce arrow after its inventor, Charles Sanders Peirce, and is a Sole sufficient operator.

The truth table for p NOR q (also written as p ↓ q, or Xpq) is as follows:

p	q	p ↓ q
T	T	F
T	F	F
F	T	F
F	F	T

The negation of a disjunction ¬(p ∨ q), and the conjunction of negations (¬p) ∧ (¬q) can be tabulated as follows:

p	q	p ∨ q	¬(p ∨ q)	¬p	¬q	(¬p) ∧ (¬q)
T	T	T	F	F	F	F
T	F	T	F	F	T	F
F	T	T	F	T	F	F
F	F	F	T	T	T	T

Inspection of the tabular derivations for NAND and NOR, under each assignment of logical values to the functional arguments p and q, produces the identical patterns of functional values for ¬(p ∧ q) as for (¬p) ∨ (¬q), and for ¬(p ∨ q) as for (¬p) ∧ (¬q). Thus the first and second expressions in each pair are logically equivalent, and may be substituted for each other in all contexts that pertain solely to their logical values.

This equivalence is one of De Morgan's laws.

Size of truth tables

If there are n input variables then there are 2ⁿ possible combinations of their truth values. A given function may produce true or false for each combination so the number of different functions of n variables is the double exponential 2²ⁿ.

n	2n	22n
0	1	2
1	2	4
2	4	16
3	8	256
4	16	65,536
5	32	4,294,967,296	≈ 4.3×109
6	64	18,446,744,073,709,551,616	≈ 1.8×1019
7	128	340,282,366,920,938,463,463,374,607,431,768,211,456	≈ 3.4×1038
8	256	115,792,089,237,316,195,423,570,985,008,687,907,853,269,984,665,640,564,039,457,584,007,913,129,639,936	≈ 1.2×1077

Truth tables for functions of three or more variables are rarely given.

Applications

Truth tables can be used to prove many other logical equivalences. For example, consider the following truth table:

${\displaystyle (p\Rightarrow q)\equiv (\mathrm{\neg }p\vee q)}$

${\displaystyle p}$	${\displaystyle q}$	${\displaystyle \mathrm{\neg }p}$	${\displaystyle \mathrm{\neg }p\vee q}$	${\displaystyle p\Rightarrow q}$
T	T	F	T	T
T	F	F	F	F
F	T	T	T	T
F	F	T	T	T

This demonstrates the fact that ${\displaystyle p\Rightarrow q}$ is logically equivalent to ${\displaystyle \mathrm{\neg }p\vee q}$.

Truth table for most commonly used logical operators

Here is a truth table that gives definitions of the 7 most commonly used out of the 16 possible truth functions of two Boolean variables P and Q:

P	Q	${\displaystyle P\wedge Q}$	${\displaystyle P\vee Q}$	${\displaystyle P\underset{\_}{\vee }Q}$	${\displaystyle P\underset{\_}{\wedge }Q}$	${\displaystyle P\Rightarrow Q}$	${\displaystyle P\Leftarrow Q}$	${\displaystyle P\iff Q}$
T	T	T	T	F	T	T	T	T
T	F	F	T	T	F	F	T	F
F	T	F	T	T	F	T	F	F
F	F	F	F	F	T	T	T	T
P	Q	${\displaystyle P\wedge Q}$	${\displaystyle P\vee Q}$	${\displaystyle P\underset{\_}{\vee }Q}$	${\displaystyle P\underset{\_}{\wedge }Q}$	${\displaystyle P\Rightarrow Q}$	${\displaystyle P\Leftarrow Q}$	${\displaystyle P\iff Q}$
		AND (conjunction)	OR (disjunction)	XOR (exclusive or)	XNOR (exclusive nor)	conditional "if-then"	conditional "then-if"	biconditional "if-and-only-if"
where    T    means true and    F    means false

Condensed truth tables for binary operators

For binary operators, a condensed form of truth table is also used, where the row headings and the column headings specify the operands and the table cells specify the result. For example, Boolean logic uses this condensed truth table notation:

∧ F T F F F T F T

∨ F T F F T T T T

This notation is useful especially if the operations are commutative, although one can additionally specify that the rows are the first operand and the columns are the second operand. This condensed notation is particularly useful in discussing multi-valued extensions of logic, as it significantly cuts down on combinatoric explosion of the number of rows otherwise needed. It also provides for quickly recognizable characteristic "shape" of the distribution of the values in the table which can assist the reader in grasping the rules more quickly.

Truth tables in digital logic

Truth tables are also used to specify the function of hardware look-up tables (LUTs) in digital logic circuitry. For an n-input LUT, the truth table will have 2^n values (or rows in the above tabular format), completely specifying a boolean function for the LUT. By representing each boolean value as a bit in a binary number, truth table values can be efficiently encoded as integer values in electronic design automation (EDA) software. For example, a 32-bit integer can encode the truth table for a LUT with up to 5 inputs.

When using an integer representation of a truth table, the output value of the LUT can be obtained by calculating a bit index k based on the input values of the LUT, in which case the LUT's output value is the kth bit of the integer. For example, to evaluate the output value of a LUT given an array of n boolean input values, the bit index of the truth table's output value can be computed as follows: if the ith input is true, let ${\displaystyle V_i=1}$, else let ${\displaystyle V_i=0}$. Then the kth bit of the binary representation of the truth table is the LUT's output value, where ${\displaystyle k=V_0\times 2^0+V_1\times 2^1+V_2\times 2^2+\cdots +V_n\times 2^n}$.

Truth tables are a simple and straightforward way to encode boolean functions, however given the exponential growth in size as the number of inputs increase, they are not suitable for functions with a large number of inputs. Other representations which are more memory efficient are text equations and binary decision diagrams.

Applications of truth tables in digital electronics

In digital electronics and computer science (fields of applied logic engineering and mathematics), truth tables can be used to reduce basic boolean operations to simple correlations of inputs to outputs, without the use of logic gates or code. For example, a binary addition can be represented with the truth table:

Binary addition

${\displaystyle A}$	${\displaystyle B}$	${\displaystyle C}$	${\displaystyle R}$
T	T	T	F
T	F	F	T
F	T	F	T
F	F	F	F

where A is the first operand, B is the second operand, C is the carry digit, and R is the result.

This truth table is read left to right:

• Value pair (A,B) equals value pair (C,R).
• Or for this example, A plus B equal result R, with the Carry C.

Note that this table does not describe the logic operations necessary to implement this operation, rather it simply specifies the function of inputs to output values.

With respect to the result, this example may be arithmetically viewed as modulo 2 binary addition, and as logically equivalent to the exclusive-or (exclusive disjunction) binary logic operation.

In this case it can be used for only very simple inputs and outputs, such as 1s and 0s. However, if the number of types of values one can have on the inputs increases, the size of the truth table will increase.

For instance, in an addition operation, one needs two operands, A and B. Each can have one of two values, zero or one. The number of combinations of these two values is 2×2, or four. So the result is four possible outputs of C and R. If one were to use base 3, the size would increase to 3×3, or nine possible outputs.

The first "addition" example above is called a half-adder. A full-adder is when the carry from the previous operation is provided as input to the next adder. Thus, a truth table of eight rows would be needed to describe a full adder's logic:

A B C* | C R
0 0 0  | 0 0
0 1 0  | 0 1
1 0 0  | 0 1
1 1 0  | 1 0
0 0 1  | 0 1
0 1 1  | 1 0
1 0 1  | 1 0
1 1 1  | 1 1

Same as previous, but..
C* = Carry from previous adder

History

Irving Anellis's research shows that C.S. Peirce appears to be the earliest logician (in 1883) to devise a truth table matrix.

From the summary of Peirce's paper:

In 1997, John Shosky discovered, on the verso of a page of the typed transcript of Bertrand Russell's 1912 lecture on "The Philosophy of Logical Atomism" truth table matrices. The matrix for negation is Russell's, alongside of which is the matrix for material implication in the hand of Ludwig Wittgenstein. It is shown that an unpublished manuscript identified as composed by Peirce in 1893 includes a truth table matrix that is equivalent to the matrix for material implication discovered by John Shosky. An unpublished manuscript by Peirce identified as having been composed in 1883–84 in connection with the composition of Peirce's "On the Algebra of Logic: A Contribution to the Philosophy of Notation" that appeared in the American Journal of Mathematics in 1885 includes an example of an indirect truth table for the conditional.

Atonality

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Atonality

"Atonal" redirects here. For the ruler of the Mixtec kingdom of Coixtlahuaca, see Atonal II.

Ending of Schoenberg's "George Lieder" Op. 15/1 presents what would be an "extraordinary" chord in tonal music, without the harmonic-contrapuntal constraints of tonal music.Duration: 5 seconds.0:05

Atonality in its broadest sense is music that lacks a tonal center, or key. Atonality, in this sense, usually describes compositions written from about the early 20th-century to the present day, where a hierarchy of harmonies focusing on a single, central triad is not used, and the notes of the chromatic scale function independently of one another. More narrowly, the term atonality describes music that does not conform to the system of tonal hierarchies that characterized European classical music between the seventeenth and nineteenth centuries. "The repertory of atonal music is characterized by the occurrence of pitches in novel combinations, as well as by the occurrence of familiar pitch combinations in unfamiliar environments".

The term is also occasionally used to describe music that is neither tonal nor serial, especially the pre-twelve-tone music of the Second Viennese School, principally Alban Berg, Arnold Schoenberg, and Anton Webern. However, "as a categorical label, 'atonal' generally means only that the piece is in the Western tradition and is not 'tonal'", although there are longer periods, e.g., medieval, renaissance, and modern modal music to which this definition does not apply. "Serialism arose partly as a means of organizing more coherently the relations used in the pre-serial 'free atonal' music. ... Thus, many useful and crucial insights about even strictly serial music depend only on such basic atonal theory".

Late 19th- and early 20th-century composers such as Alexander Scriabin, Claude Debussy, Béla Bartók, Paul Hindemith, Sergei Prokofiev, Igor Stravinsky, and Edgard Varèse have written music that has been described, in full or in part, as atonal.

History

While music without a tonal center had been previously written, for example Franz Liszt's Bagatelle sans tonalité of 1885, it is with the coming of the twentieth century that the term atonality began to be applied to pieces, particularly those written by Arnold Schoenberg and The Second Viennese School. The term "atonality" was coined in 1907 by Joseph Marx in a scholarly study of tonality, which was later expanded into his doctoral thesis.

Their music arose from what was described as the "crisis of tonality" between the late nineteenth century and early twentieth century in classical music. This situation had arisen over the course of the nineteenth century due to the increasing use of

ambiguous chords, improbable harmonic inflections, and more unusual melodic and rhythmic inflections than what was possible within the styles of tonal music. The distinction between the exceptional and the normal became more and more blurred. As a result, there was a "concomitant loosening" of the synthetic bonds through which tones and harmonies had been related to one another. The connections between harmonies were uncertain even on the lowest chord-to-chord level. On higher levels, long-range harmonic relationships and implications became so tenuous, that they hardly functioned at all. At best, the felt probabilities of the style system had become obscure. At worst, they were approaching a uniformity, which provided few guides for either composition or listening. 

The first phase, known as "free atonality" or "free chromaticism", involved a conscious attempt to avoid traditional diatonic harmony. Works of this period include the opera Wozzeck (1917–1922) by Alban Berg and Pierrot lunaire (1912) by Schoenberg.

The second phase, begun after World War I, was exemplified by attempts to create a systematic means of composing without tonality, most famously the method of composing with 12 tones or the twelve-tone technique. This period included Berg's Lulu and Lyric Suite, Schoenberg's Piano Concerto, his oratorio Die Jakobsleiter and numerous smaller pieces, as well as his last two string quartets. Schoenberg was the major innovator of the system. His student, Anton Webern, however, is anecdotally claimed to have begun linking dynamics and tone color to the primary row, making rows not only of pitches but of other aspects of music as well. However, actual analysis of Webern's twelve-tone works has so far failed to demonstrate the truth of this assertion. One analyst concluded, following a minute examination of the Piano Variations, op. 27, that

while the texture of this music may superficially resemble that of some serial music ... its structure does not. None of the patterns within separate nonpitch characteristics makes audible (or even numerical) sense in itself. The point is that these characteristics are still playing their traditional role of differentiation.

Twelve-tone technique, combined with the parametrization (separate organization of four aspects of music: pitch, attack character, intensity, and duration) of Olivier Messiaen, would be taken as the inspiration for serialism.

Atonality emerged as a pejorative term to condemn music in which chords were organized seemingly with no apparent coherence. In Nazi Germany, atonal music was attacked as "Bolshevik" and labeled as degenerate (Entartete Musik) along with other music produced by enemies of the Nazi regime. Many composers had their works banned by the regime, not to be played until after its collapse at the end of World War II.

After Schoenberg's death, Igor Stravinsky used the twelve-tone technique. Iannis Xenakis generated pitch sets from mathematical formulae, and also saw the expansion of tonal possibilities as part of a synthesis between the hierarchical principle and the theory of numbers, principles which have dominated music since at least the time of Parmenides.

Free atonality

The twelve-tone technique was preceded by Schoenberg's freely atonal pieces of 1908 to 1923, which, though free, often have as an "integrative element...a minute intervallic cell" that in addition to expansion may be transformed as with a tone row, and in which individual notes may "function as pivotal elements, to permit overlapping statements of a basic cell or the linking of two or more basic cells".

The twelve-tone technique was also preceded by nondodecaphonic serial composition used independently in the works of Alexander Scriabin, Igor Stravinsky, Béla Bartók, Carl Ruggles, and others. "Essentially, Schoenberg and Hauer systematized and defined for their own dodecaphonic purposes a pervasive technical feature of 'modern' musical practice, the ostinato."

Composing atonal music

Setting out to compose atonal music may seem complicated because of both the vagueness and generality of the term. Additionally George Perle explains that, "the 'free' atonality that preceded dodecaphony precludes by definition the possibility of self-consistent, generally applicable compositional procedures". However, he provides one example as a way to compose atonal pieces, a pre-twelve-tone technique piece by Anton Webern, which rigorously avoids anything that suggests tonality, to choose pitches that do not imply tonality. In other words, reverse the rules of the common practice period so that what was not allowed is required and what was required is not allowed. This is what was done by Charles Seeger in his explanation of dissonant counterpoint, which is a way to write atonal counterpoint.

Opening of Schoenberg's Klavierstück, Op. 11, No. 1, exemplifying his four procedures as listed by Kostka & Payne 1995Duration: 10 seconds.0:10

Kostka and Payne list four procedures as operational in the atonal music of Schoenberg, all of which may be taken as negative rules. Avoidance of melodic or harmonic octaves, avoidance of traditional pitch collections such as major or minor triads, avoidance of more than three successive pitches from the same diatonic scale, and use of disjunct melodies (avoidance of conjunct melodies).

Further, Perle agrees with Oster and Katz that, "the abandonment of the concept of a root-generator of the individual chord is a radical development that renders futile any attempt at a systematic formulation of chord structure and progression in atonal music along the lines of traditional harmonic theory". Atonal compositional techniques and results "are not reducible to a set of foundational assumptions in terms of which the compositions that are collectively designated by the expression 'atonal music' can be said to represent 'a system' of composition". Equal-interval chords are often of indeterminate root, mixed-interval chords are often best characterized by their interval content, while both lend themselves to atonal contexts.

Perle also points out that structural coherence is most often achieved through operations on intervallic cells. A cell "may operate as a kind of microcosmic set of fixed intervallic content, statable either as a chord or as a melodic figure or as a combination of both. Its components may be fixed with regard to order, in which event it may be employed, like the twelve-tone set, in its literal transformations. … Individual tones may function as pivotal elements, to permit overlapping statements of a basic cell or the linking of two or more basic cells".

Regarding the post-tonal music of Perle, one theorist wrote: "While ... montages of discrete-seeming elements tend to accumulate global rhythms other than those of tonal progressions and their rhythms, there is a similarity between the two sorts of accumulates spatial and temporal relationships: a similarity consisting of generalized arching tone-centers linked together by shared background referential materials".

Another approach of composition techniques for atonal music is given by Allen Forte who developed the theory behind atonal music. Forte describes two main operations: transposition and inversion. Transposition can be seen as a rotation of t either clockwise or anti-clockwise on a circle, where each note of the chord is rotated equally. For example, if t = 2 and the chord is [0 3 6], transposition (clockwise) will be [2 5 8]. Inversion can be seen as a symmetry with respect to the axis formed by 0 and 6. If we carry on with our example [0 3 6] becomes [0 9 6].

An important characteristic are the invariants, which are the notes which stay identical after a transformation. No difference is made between the octave in which the note is played so that, for example, all C♯s are equivalent, no matter the octave in which they actually occur. This is why the 12-note scale is represented by a circle. This leads us to the definition of the similarity between two chords which considers the subsets and the interval content of each chord.

Reception and legacy

Controversy over the term itself

The term "atonality" itself has been controversial. Arnold Schoenberg, whose music is generally used to define the term, was vehemently opposed to it, arguing that "The word 'atonal' could only signify something entirely inconsistent with the nature of tone... to call any relation of tones atonal is just as farfetched as it would be to designate a relation of colors aspectral or acomplementary. There is no such antithesis".

Composer and theorist Milton Babbitt also disparaged the term, saying "The works that followed, many of them now familiar, include the Five Pieces for Orchestra, Erwartung, Pierrot Lunaire, and they and a few yet to follow soon were termed 'atonal,' by I know not whom, and I prefer not to know, for in no sense does the term make sense. Not only does the music employ 'tones,' but it employs precisely the same 'tones,' the same physical materials, that music had employed for some two centuries. In all generosity, 'atonal' may have been intended as a mildly analytically derived term to suggest 'atonic' or to signify 'a-triadic tonality', but, even so there were infinitely many things the music was not".

"Atonal" developed a certain vagueness in meaning as a result of its use to describe a wide variety of compositional approaches that deviated from traditional chords and chord progressions. Attempts to solve these problems by using terms such as "pan-tonal", "non-tonal", "multi-tonal", "free-tonal" and "without tonal center" instead of "atonal" have not gained broad acceptance.

Criticism of the concept of atonality

Composer Anton Webern held that "new laws asserted themselves that made it impossible to designate a piece as being in one key or another". Composer Walter Piston, on the other hand, said that, out of long habit, whenever performers "play any little phrase they will hear it in some key—it may not be the right one, but the point is they will play it with a tonal sense. ... [T]he more I feel I know Schoenberg's music the more I believe he thought that way himself. ... And it isn't only the players; it's also the listeners. They will hear tonality in everything".

Donald Jay Grout similarly doubted whether atonality is really possible, because "any combination of sounds can be referred to a fundamental root". He defined it as a fundamentally subjective category: "atonal music is music in which the person who is using the word cannot hear tonal centers".

One difficulty is that even an otherwise "atonal" work, tonality "by assertion" is normally heard on the thematic or linear level. That is, centricity may be established through the repetition of a central pitch or from emphasis by means of instrumentation, register, rhythmic elongation, or metric accent.

Criticism of atonal music

Swiss conductor, composer, and musical philosopher Ernest Ansermet, a critic of atonal music, wrote extensively on this in the book Les fondements de la musique dans la conscience humaine (The Foundations of Music in Human Consciousness), where he argued that the classical musical language was a precondition for musical expression with its clear, harmonious structures. Ansermet argued that a tone system can only lead to a uniform perception of music if it is deduced from just a single interval. For Ansermet this interval is the fifth.

Examples

An example of atonal music would be Arnold Schoenberg’s “Pierrot Lunaire”, which is a song cycle composed in 1912. The work uses a technique called “Sprechstimme” or spoken singing, and the music is atonal, meaning that there is no clear tonal center or key. Instead, the notes of the chromatic scale function independently of each other, and the harmonies do not follow the traditional tonal hierarchy found in classical music. The result is a dissonant and jarring sound that is quite different from the harmonies found in tonal music.