A two-graph 
on nodes 
is a collection 
of unordered triples of the vertices (the so-called "odd triples") such
that each 4-tuple of 
contains an even number of elements of 
as subsets.
Given a graph on 
, let 
be the set of triples spanning an odd number of edges.
Seidel switching preserves this parity,
so every graph in a switching class determines
the same two-graph. Conversely, all graphs determining a given two-graph belong to
a single switching class (Mallows and Sloane 1975).
