Because the result of performing the operation on a pair of elements of is again an element of , the operation is called a closed (or internal) binary operation on (or sometimes expressed as having the property of closure).
On the set of real numbers , is a binary operation since the sum of two real numbers is a real number.
On the set of natural numbers , is a binary operation since the sum of two natural numbers is a natural number. This is a different binary operation than the previous one since the sets are different.
On the set of matrices with real entries, is a binary operation since the sum of two such matrices is a matrix.
On the set of matrices with real entries, is a binary operation since the product of two such matrices is a matrix.
For a given set , let be the set of all functions . Define by for all , the composition of the two functions and in . Then is a binary operation since the composition of the two functions is again a function on the set (that is, a member of ).
The first three examples above are commutative and all of the above examples are associative.
On the set of real numbers , subtraction, that is, , is a binary operation which is not commutative since, in general, . It is also not associative, since, in general, ; for instance, but .
On the set of natural numbers , the binary operation exponentiation, , is not commutative since, (cf. Equation xy = yx), and is also not associative since . For instance, with , , and , , but . By changing the set to the set of integers , this binary operation becomes a partial binary operation since it is now undefined when and is any negative integer. For either set, this operation has a right identity (which is ) since for all in the set, which is not an identity (two sided identity) since in general.
Division (), a partial binary operation on the set of real or rational numbers, is not commutative or associative. Tetration (), as a binary operation on the natural numbers, is not commutative or associative and has no identity element.
Binary operations are often written using infix notation such as , , or (by juxtaposition with no symbol) rather than by functional notation of the form . Powers are usually also written without operator, but with the second argument as superscript.
Binary operations are sometimes written using prefix or (more frequently) postfix notation, both of which dispense with parentheses. They are also called, respectively, Polish notation and reverse Polish notation.
Binary operations as ternary relationsEdit
A binary operation on a set may be viewed as a ternary relation on , that is, the set of triples in for all and in .
External binary operationsEdit
An external binary operation is a binary function from to . This differs from a binary operation on a set in the sense in that need not be ; its elements come from outside.
Some external binary operations may alternatively be viewed as an action of on . This requires the existence of an associative multiplication in , and a compatibility rule of the form , where and (here, both the external operation and the multiplication in are denoted by juxtaposition).
The dot product of two vectors maps to , where is a field and is a vector space over . It depends on authors whether it is considered as a binary operation.