The Grünbaum-Rigby configuration is a 
configuration consisting
of 21 points and 21 lines such that four points lie on each line and four lines pass
through each point (left figure above). It was first described by Klein (1879), but
a geometric realization in the Euclidean plane
was not found until Grünbaum and Rigby (1990) constructed one by overlaying
three regular heptagrams. Note that the points also
determine seven additional lines through three points (illustrated in red in the
right figure above) which are not however part of the configuration.
The Levi graph of the Grünbaum-Rigby configuration may be termed the Grünbaum-Rigby graph.
The best known solution to the orchard-planting problem for 22 points with 4 points per line is obtained by a adding a point at the center of the configuration through which opposite vertices of the configuration lie, yielding 28 lines (E. Pegg, Jr., pers. comm., Dec. 11, 2023). Note that this arrangement is not a configuration since while five lines pass through 21 of the points, seven lines pass through the central point.
)." J. London Math. Soc. 41, 336-348,
1990.Grünbaum, B.