Ordered Pair

Let $x$ and $y$ be objects. Write $\{x\}$ for the set whose only element is $x$, and $\{x,y\}$ for the set whose elements are exactly $x$ and $y$.

The ordered pair of $x$ and $y$ is the set

$$(x,y)=\bigl\{\,\{x\},\ \{x,y\}\,\bigr\}.$$

In this expression $x$ is the first component and $y$ is the second component.