information:extraction, structural:expansion
Wiener's definition

Norbert Wiener proposed the first set theoretical definition of the ordered pair in 1914: (a, b):= { { { a }, ∅ }, { { b } } }. {\displaystyle \left(a,b\right):=\left\{\left\{\left\{a\right\},\,\emptyset \right\},\,\left\{\left\{b\right\}\right\}\right\}.} He observed that this definition made it possible to define the types of Principia Mathematica as sets. Principia Mathematica had taken types, and hence relations of all arities, as primitive.

Wiener used {{ b }} instead of { b } to make the definition compatible with type theory where all elements in a class must be of the same "type". With b nested within an additional set, its type is equal to { { a }, ∅ } {\displaystyle \{\{a\},\emptyset \}} 's.