A ***set*** is a collection of individual items called *elements* or *members* If $A$ and $B$ are two sets, $A$ is a ***subset*** of $B$ if all members of $A$ are in $B$. If $B$ consists of other elements than those in $A$, then $A$ is a ***proper*** subset. $ A\subseteq B\quad (\text{subset}) $ $ A\subset B \quad (\text{proper subset}) $