20_4 Configuration

A configuration is an abstract configuration of 20 points and 20 lines, with exactly four lines incident with each point and exactly four points incident with each line. Two distinct points are incident with at most one common line. A geometric realization represents these incidences by points and straight lines in the real projective plane, without adding incidences between the selected points and lines.

The Grünbaum 20₄ configuration is a real geometric example constructed by Grünbaum (2008). Its construction disproved the conjecture of Grünbaum and Rigby (1990) that no geometric n₄ configuration exists for . This named example is not a classification of all configurations.