Mondrian's neoplastic painting shows a transition from a covert to an ostensive treatment of rhythm. The transition roughly divides Mondrian's practical development into two periods: the first from 1919 to 1932, and the next covering the "double-line' and New York periods of 1932 to 1944. The transition in Mondrian's theoretical development, however, which this Chapter investigates through analyses of Mondrian's own writing, indicates a shift which occurs a few years earlier than its practical counterpart: the two periods which divide Mondrian's theoretical development occur, roughly, from 1917 to 1927, the first stage of his early mature painting period, and then from 1927 to 1932 (before the "double-line' was introduced in 1932), which is the second early mature painting period. Finally, the years 1932 to 1944 mark the period of dynamic (kinetic) rhythm, and includes the New York period of 1940 to 1944.