Grünbaum–Rigby configuration

Originally studied by Felix Klein in the complex projective plane in connection with the Klein quartic, it was first realized in the Euclidean plane by Branko Grünbaum and John F. Rigby.

[2] In Klein's description, these points and lines belong to the complex projective plane, a space whose coordinates are complex numbers rather than the real-number coordinates of the Euclidean plane.

The geometric realisation of this configuration as points and lines in the Euclidean plane, based on overlaying three regular heptagrams, was only established much later, by Branko Grünbaum and J. F. Rigby (1990).

Their paper on it became the first of a series of works on configurations by Grünbaum, and contained the first published graphical depiction of a 4-configuration.

However, the notation does not specify the configuration itself, only its type (the numbers of points, lines, and incidences).

It also does not specify whether the configuration is purely combinatorial (an abstract incidence pattern of lines and points) or whether the points and lines of the configuration are realizable in the Euclidean plane or another standard geometry.

[4] The Grünbaum–Rigby configuration can be constructed from the seven points of a regular heptagon and its 14 interior diagonals.