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.
Two-Graph
See also
Eulerian GraphExplore with Wolfram|Alpha
References
Bussemaker, F. C.; Mathon, R. A.; and Seidel, J. J. "Tables of Two-Graphs." In Combinatorics and Graph Theory (Ed. S. B. Rao). Berlin: Springer-Verlag, pp. 70-112, 1981.Mallows, C. L. and Sloane, N. J. A. "Two-Graphs, Switching Classes, and Euler Graphs are Equal in Number." SIAM J. Appl. Math. 28, 876-880, 1975.Spence, E. "Two-Graphs." Ch. VI.6 in Colbourn, C. J. and Dinitz, J. H. (Eds.). CRC Handbook of Combinatorial Designs. Boca Raton, FL: CRC Press, pp. 686-694, 1996.Referenced on Wolfram|Alpha
Two-GraphCite this as:
Weisstein, Eric W. "Two-Graph." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Two-Graph.html