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).
