Art of M.C. Escher, Page 3

In the winter of 1937-1938, Escher started to assemble notebooks of his obsession with repeating geometric patterns.  He sought to develop a "layman's theory" that would describe the rules of filling the plane with repeating patterns.  He developed his theory and, in 1941 to 1942, assembled two notebooks: one containing descriptions, diagrams, and terms; the other a collection of finished drawings.  These drawings would influence all of Escher's work from then on (Schattschneider, D., 2004, pp.36 & 56).