Definition:Subset

Definition
A set $S$ is a subset of a set $T$ iff all of the elements of $S$ are also elements of $T$, and it is written $S \subseteq T$.

If the elements of $S$ are not all also elements of $T$, then $S$ is not a subset of $T$:


 * $S \nsubseteq T$ means $\neg \left( {S \subseteq T}\right)$

For example, if $S = \left\{ {1, 2, 3} \right\}$ and $T = \left\{ {1, 2, 3, 4} \right\}$, then $S \subseteq T$.

So, if we can prove that if an element is in $S$ then it is also in $T$, then we have proved that $S$ is a subset of $T$.

That is:
 * $S \subseteq T \iff \forall x: \left({ x \in S \implies x \in T}\right)$

In class-set theories in which sets are classes, a set $S$ is a subset of a class $T$ iff it is a subclass of $T$.

In class-set theories in which sets are not classes, writers may nevertheless say that a set $S$ is a subset of a class $T$ iff every element of $S$ is also an element of $T$.

Superset
If $S$ is a subset of $T$, then that means $T$ is a superset of $S$, which can be expressed by the notation $T \supseteq S$. This can be interpreted as $T$ contains $S$.

Thus $S \subseteq T$ and $T \supseteq S$ mean the same thing.

Also known as
$S \subseteq T$ can also be read as:
 * $S$ is contained in $T$, or $T$ contains $S$
 * $S$ is included in $T$, or $T$ includes $S$

Hence $\subseteq$ is also called the inclusion relation, or (more rarely) the containment relation.

Notation in the literature can be confusing.

Many authors, for example and, use $\subset$.

If it is important with this usage to indicate that $S$ is a proper subset of $T$, the notation $S \subsetneq T$ or $T \supsetneq S$ can be used.

Beware: "$T$ contains $S$" can be used to mean that $S$ is an element of $T$.

Also see

 * Definition:Proper Subset