Two graphs on the same vertex set are switching equivalent if one can be obtained from the other by Seidel switching. A switching class is an equivalence class under this relation (Mallows and Sloane 1975).
Since Seidel switching preserves the parity of the number of edges spanned by every triple of vertices, a switching class determines a two-graph 
whose elements are the triples
spanning an odd number of edges. Conversely, graphs determining the same two-graph
are switching equivalent, so each two-graph determines exactly one switching class
(Mallows and Sloane 1975).
