ABSTRACT
The crossing number is the smallest number of pairwise crossings of edges. Starting with this phrasing it is hardly surprising that early on the crossing number was sometimes defined as the smallest number of pairwise crossing edges, or to be more precise, the smallest number of pairs of edges crossing in a drawing. This may not sound too different from the usual definition of the crossing number—and sometimes has been used to define the crossing number—but we know remarkably little about this new notion, the pair crossing number, and how it relates to the standard crossing number.
