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