M.C. Escher 8
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Also in the list was a paper by professor F. Haag in which the definitions of the division of the plane were discussed together with drawings and notations. 'Regular divisions of the plane consist of congruent, convex polygons joined together, the arrangement by which the polygons join each other is the same throughout.'

This sparked a frenzy of drawing in his notebooks. He proved the word convex was not necessary. He began his own 'Layman's Theory' and thereafter classified all his drawings based on it. Nowadays, the most often used is the Union of Crystallography's system - see 'Tessellations - symmetry'.