The ordered pair associated to $A$ and $B$ is the set $\webleft (A,B\webright )$ defined by
\[ \webleft (A,B\webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ \left\{ A\right\} ,\left\{ A,B\right\} \right\} . \]