Associative property
This article needs additional citations for verification. (June 2009) 
Transformation rules 

Propositional calculus 
Rules of inference 
Rules of replacement 

Predicate logic 
In mathematics, the associative property^{} is a property of some binary operations. In propositional logic, associativity is a valid rule of replacement for expressions in logical proofs.
Within an expression containing two or more occurrences in a row of the same associative operator, the order in which the operations are performed does not matter as long as the sequence of the operands is not changed. That is, rearranging the parentheses in such an expression will not change its value. Consider the following equations:
Even though the parentheses were rearranged, the values of the expressions were not altered. Since this holds true when performing addition and multiplication on any real numbers, it can be said that "addition and multiplication of real numbers are associative operations".
Associativity is not to be confused with commutativity, which addresses whether a × b = b × a.
Associative operations are abundant in mathematics; in fact, many algebraic structures (such as semigroups and categories) explicitly require their binary operations to be associative.
However, many important and interesting operations are nonassociative; some examples include subtraction, exponentiation and the vector cross product. In contrast to the theoretical counterpart, the addition of floating point numbers in computer science is not associative, and is an important source of rounding error.
Definition
Formally, a binary operation ∗ on a set S is called associative if it satisfies the associative law:
 (x ∗ y) ∗ z = x ∗ (y ∗ z) for all x, y, z in S.
Here, ∗ is used to replace the symbol of the operation, which may be any symbol, and even the absence of symbol like for the multiplication.
 (xy)z = x(yz) = xyz for all x, y, z in S.
The associative law can also be expressed in functional notation thus: f(f(x, y), z) = f(x, f(y, z)).
Generalized associative law
If a binary operation is associative, repeated application of the operation produces the same result regardless how valid pairs of parenthesis are inserted in the expression.^{} This is called the generalized associative law. For instance, a product of four elements may be written in five possible ways:
 ((ab)c)d
 (ab)(cd)
 (a(bc))d
 a((bc)d)
 a(b(cd))
If the product operation is associative, the generalized associative law says that all these formulas will yield the same result, making the parenthesis unnecessary. Thus "the" product can be written unambiguously as
 abcd.
As the number of elements increases, the number of possible ways to insert parentheses grows quickly, but they remain unnecessary for disambiguation.
Examples
Some examples of associative operations include the following.
 The concatenation of the three strings
"hello"
," "
,"world"
can be computed by concatenating the first two strings (giving"hello "
) and appending the third string ("world"
), or by joining the second and third string (giving" world"
) and concatenating the first string ("hello"
) with the result. The two methods produce the same result; string concatenation is associative (but not commutative).
 In arithmetic, addition and multiplication of real numbers are associative; i.e.,

 Because of associativity, the grouping parentheses can be omitted without ambiguity.
 Addition and multiplication of complex numbers and quaternions are associative. Addition of octonions is also associative, but multiplication of octonions is nonassociative.
 The greatest common divisor and least common multiple functions act associatively.
 Taking the intersection or the union of sets:
 If M is some set and S denotes the set of all functions from M to M, then the operation of functional composition on S is associative:
 Slightly more generally, given four sets M, N, P and Q, with h: M to N, g: N to P, and f: P to Q, then
 as before. In short, composition of maps is always associative.
 Consider a set with three elements, A, B, and C. The following operation:

× A B C A A A A B A B C C A A A
 is associative. Thus, for example, A(BC)=(AB)C = A. This operation is not commutative.
 Because matrices represent linear transformation functions, with matrix multiplication representing functional composition, one can immediately conclude that matrix multiplication is associative.
Propositional logic
Rule of replacement
In standard truthfunctional propositional logic, association,^{}^{} or associativity^{} are two valid rules of replacement. The rules allow one to move parentheses in logical expressions in logical proofs. The rules are:
and
where "" is a metalogical symbol representing "can be replaced in a proof with."
Truth functional connectives
Associativity is a property of some logical connectives of truthfunctional propositional logic. The following logical equivalences demonstrate that associativity is a property of particular connectives. The following are truthfunctional tautologies.
Associativity of disjunction:
Associativity of conjunction:
Associativity of equivalence:
Nonassociativity
A binary operation on a set S that does not satisfy the associative law is called nonassociative. Symbolically,
For such an operation the order of evaluation does matter. For example:
Also note that infinite sums are not generally associative, for example:
whereas
The study of nonassociative structures arises from reasons somewhat different from the mainstream of classical algebra. One area within nonassociative algebra that has grown very large is that of Lie algebras. There the associative law is replaced by the Jacobi identity. Lie algebras abstract the essential nature of infinitesimal transformations, and have become ubiquitous in mathematics.
There are other specific types of nonassociative structures that have been studied in depth; these tend to come from some specific applications or areas such as combinatorial mathematics. Other examples are Quasigroup, Quasifield, Nonassociative ring, Nonassociative algebra and Commutative nonassociative magmas.
Nonassociativity of floating point calculation
In mathematics, addition and multiplication of real numbers is associative. By contrast, in computer science, the addition and multiplication of floating point numbers is not associative, as rounding errors are introduced when dissimilarsized values are joined together.^{}
To illustrate this, consider a floating point representation with a 4bit mantissa:
(1.000_{2}×2^{0} + 1.000_{2}×2^{0}) + 1.000_{2}×2^{4} = 1.000_{2}×2^{1} + 1.000_{2}×2^{4} = 1.001_{2}×2^{4}
1.000_{2}×2^{0} + (1.000_{2}×2^{0} + 1.000_{2}×2^{4}) = 1.000_{2}×2^{0} + 1.000_{2}×2^{4} = 1.000_{2}×2^{4}
Even though most computers compute with a 24 or 53 bits of mantissa,^{} this is an important source of rounding error, and approaches such as the Kahan Summation Algorithm are ways to minimise the errors. It can be especially problematic in parallel computing.^{}^{}
Notation for nonassociative operations
In general, parentheses must be used to indicate the order of evaluation if a nonassociative operation appears more than once in an expression. However, mathematicians agree on a particular order of evaluation for several common nonassociative operations. This is simply a notational convention to avoid parentheses.
A leftassociative operation is a nonassociative operation that is conventionally evaluated from left to right, i.e.,
while a rightassociative operation is conventionally evaluated from right to left:
Both leftassociative and rightassociative operations occur. Leftassociative operations include the following:
 Subtraction and division of real numbers:
 Function application:

 This notation can be motivated by the currying isomorphism.
Rightassociative operations include the following:
 Exponentiation of real numbers:
 The reason exponentiation is rightassociative is that a repeated leftassociative exponentiation operation would be less useful. Multiple appearances could (and would) be rewritten with multiplication:
 Using rightassociative notation for these operations can be motivated by the CurryHoward correspondence and by the currying isomorphism.
Nonassociative operations for which no conventional evaluation order is defined include the following.
 Taking the Cross product of three vectors:
 Taking the pairwise average of real numbers:
 Taking the relative complement of sets is not the same as . (Compare material nonimplication in logic.)
See also
 Light's associativity test
 A semigroup is a set with a closed associative binary operation.
 Commutativity and distributivity are two other frequently discussed properties of binary operations.
 Power associativity, alternativity and Nary associativity are weak forms of associativity.