The pairing of $X$ and $Y$ is the set $\left\{ X,Y\right\} $ defined by
\[ \left\{ X,Y\right\} \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ x\in A\ \middle |\ \text{$x=X$ or $x=Y$}\right\} , \]
where $A$ is the set in the axiom of pairing, 
of .
