The figure 21 in the title of Hans Richter’s film is not necessarily indicative of the date of origin of this milestone in abstract moving images, which has been lost in the mists of history. The fact that “Rhythmus 21” was screened at the matinee “The Absolute Film” in Berlin in 1925, however, is documented. Rectangles move through space on fixed paths, growing and shrinking to create an illusion of depth.