In mathematics, set *A* is a **subset** of a set *B* if all elements of *A* are also elements of *B*; *B* is then a **superset** of *A*. It is possible for *A* and *B* to be equal; if they are unequal, then *A* is a **proper subset** of *B*. The relationship of one set being a subset of another is called **inclusion** (or sometimes **containment**). *A* is a subset of *B* may also be expressed as *B* includes (or contains) *A* or *A* is included (or contained) in *B*. A ** k-subset** is a subset with

The subset relation defines a partial order on sets. In fact, the subsets of a given set form a Boolean algebra under the subset relation, in which the join and meet are given by intersection and union, and the subset relation itself is the Boolean inclusion relation.

If *A* and *B* are sets and every element of *A* is also an element of *B*, then:

*A*is a**subset**of*B*, denoted by , or equivalently,*B*is a**superset**of*A*, denoted by

If *A* is a subset of *B*, but *A* is not equal to *B* (i.e. there exists at least one element of B which is not an element of *A*), then:

*A*is a**proper**(or**strict**)**subset**of*B*, denoted by , or equivalently,*B*is a**proper**(or**strict**)**superset**of*A*, denoted by .

The empty set, written or is a subset of any set *X* and a proper subset of any set except itself, the inclusion relation is a partial order on the set (the power set of *S*—the set of all subsets of *S*^{[1]}) defined by . We may also partially order by reverse set inclusion by defining

When quantified, is represented as ^{[2]}

We can prove the statement by applying a proof technique known as the element argument^{[3]}:

Let sets

AandBbe given. To prove that

supposethatais a particular but arbitrarily chosen element of Ashowthatais an element ofB.

The validity of this technique can be seen as a consequence of Universal generalization: the technique shows for an arbitrarily chosen element *c*. Universal generalisation then implies which is equivalent to as stated above.

The set of all subsets of is called its powerset, and is denoted by . The set of all -subsets of is denoted by , in analogue with the notation for binomial coefficients, which count the number of -subsets of an -element set. In set theory, the notation is also common, especially when is a transfinite cardinal number.

- A set
*A*is a**subset**of*B*if and only if their intersection is equal to A.

- Formally:

- A set
*A*is a**subset**of*B*if and only if their union is equal to B.

- Formally:

- A
**finite**set*A*is a**subset**of*B*, if and only if the cardinality of their intersection is equal to the cardinality of A.

- Formally:

Some authors use the symbols and to indicate *subset* and *superset* respectively; that is, with the same meaning as and instead of the symbols and ^{[4]} For example, for these authors, it is true of every set *A* that

Other authors prefer to use the symbols and to indicate *proper* (also called strict) subset and *proper* superset respectively; that is, with the same meaning as and instead of the symbols and ^{[5]} This usage makes and analogous to the inequality symbols and For example, if then *x* may or may not equal *y*, but if then *x* definitely does not equal *y*, and *is* less than *y*. Similarly, using the convention that is proper subset, if then *A* may or may not equal *B*, but if then *A* definitely does not equal *B*.

- The set A = {1, 2} is a proper subset of B = {1, 2, 3}, thus both expressions and are true.
- The set D = {1, 2, 3} is a subset (but
*not*a proper subset) of E = {1, 2, 3}, thus is true, and is not true (false). - Any set is a subset of itself, but not a proper subset. ( is true, and is false for any set X.)
- The set {
*x*:*x*is a prime number greater than 10} is a proper subset of {*x*:*x*is an odd number greater than 10} - The set of natural numbers is a proper subset of the set of rational numbers; likewise, the set of points in a line segment is a proper subset of the set of points in a line. These are two examples in which both the subset and the whole set are infinite, and the subset has the same cardinality (the concept that corresponds to size, that is, the number of elements, of a finite set) as the whole; such cases can run counter to one's initial intuition.
- The set of rational numbers is a proper subset of the set of real numbers. In this example, both sets are infinite, but the latter set has a larger cardinality (or
*power*) than the former set.

Another example in an Euler diagram:

Inclusion is the canonical partial order, in the sense that every partially ordered set is isomorphic to some collection of sets ordered by inclusion. The ordinal numbers are a simple example: if each ordinal *n* is identified with the set of all ordinals less than or equal to *n*, then if and only if

For the power set of a set *S*, the inclusion partial order is—up to an order isomorphism—the Cartesian product of (the cardinality of *S*) copies of the partial order on for which This can be illustrated by enumerating , and associating with each subset (i.e., each element of ) the *k*-tuple from of which the *i*th coordinate is 1 if and only if is a member of *T*.