The Gray configuration is a configuration consisting
of 27 points and 27 lines, with each point lying on exactly three lines and each
line passing through exactly three points. It can be realized by taking the 27 points
of a grid graph and the 27 lines through these points
that are parallel to one of the coordinate axes of the grid.

Boben, M. and Pisanski, T. "Polycyclic Configurations." European J. Combin.24, 431-457, 2003.Hladnik, M.; Marušič,
D.; and Pisanski, T. "Cyclic Haar Graphs." Discr. Math.244,
137-152, 2002.Marušič, D.; Pisanski, T.; and Wilson, S.
"The Genus of the GRAY [sic] Graph is 7." Europ. J. Combin.26,
377-385, 2005.