In mathematics, a 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 k elements.

Euler diagram showing
A is a subset of B (denoted ) and, conversely, B is a superset of A (denoted ).

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 A and B be given. To prove that  

  1. suppose that a is a particular but arbitrarily chosen element of A
  2. show that a is an element of B.

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

⊂ and ⊃ symbols


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   (a reflexive relation).

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 (an irreflexive relation). 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.

Examples of subsets

The regular polygons form a subset of the polygons.
  • 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:

Other properties of inclusion

  and   implies  

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 ith coordinate is 1 if and only if   is a member of T.

See also

  • Convex subset – In geometry, set whose intersection with every line is a single line segment
  • Inclusion order – Partial order that arises as the subset-inclusion relation on some collection of objects
  • Region – Connected open subset of a topological space
  • Subset sum problem – Decision problem in computer science
  • Subsumptive containment – System of elements that are subordinated to each other
  • Total subset – Subset T of a topological vector space X where the linear span of T is a dense subset of X
  • Mereology – Study of parts and the wholes they form


  1. ^ Weisstein, Eric W. "Subset". Retrieved 2020-08-23.
  2. ^ Rosen, Kenneth H. (2012). Discrete Mathematics and Its Applications (7th ed.). New York: McGraw-Hill. p. 119. ISBN 978-0-07-338309-5.
  3. ^ Epp, Susanna S. (2011). Discrete Mathematics with Applications (Fourth ed.). p. 337. ISBN 978-0-495-39132-6.
  4. ^ Rudin, Walter (1987), Real and complex analysis (3rd ed.), New York: McGraw-Hill, p. 6, ISBN 978-0-07-054234-1, MR 0924157
  5. ^ Subsets and Proper Subsets (PDF), archived from the original (PDF) on 2013-01-23, retrieved 2012-09-07