A significant modification of Cubism between 1914 and 1916 was signaled by a shift towards a strong emphasis on large overlapping geometric planes and flat surface activity. This grouping of styles of painting and sculpture, especially significant between 1917 and 1920, was practiced by several artists; particularly those under contract with the art dealer and collector Léonce Rosenberg. The tightening of the compositions, the clarity and sense of order reflected in these works, led to its being referred to by the critic Maurice Raynal (fr) as 'crystal' Cubism. Considerations manifested by Cubists prior to the outset of World War I—such as the fourth dimension, dynamism of modern life, the occult, and Henri Bergson's concept of duration—had now been vacated, replaced by a purely formal frame of reference.
What was the second time period that a significant change began to happen in Cubism?
between 1917 and 1920