# Definition:Set Equality/Definition 2

## Definition

Let $S$ and $T$ be sets.

Then $S$ and $T$ are equal if and only if:

$S$ is a subset of $T$

and

$T$ is a subset of $S$

## Notation

This can be denoted in several ways:

$S = T \iff \paren {S \subseteq T} \land \paren {T \subseteq S}$

or:

$S = T \iff \paren {S \subseteq T} \land \paren {S \supseteq T}$

or:

$S = T \iff S \subseteq T \subseteq S$