The historical study of Cubism began in the late 1920s, drawing at first from sources of limited data, namely the opinions of Guillaume Apollinaire. It came to rely heavily on Daniel-Henry Kahnweiler's book Der Weg zum Kubismus (published in 1920), which centered on the developments of Picasso, Braque, Léger, and Gris. The terms "analytical" and "synthetic" which subsequently emerged have been widely accepted since the mid-1930s. Both terms are historical impositions that occurred after the facts they identify. Neither phase was designated as such at the time corresponding works were created. "If Kahnweiler considers Cubism as Picasso and Braque," wrote Daniel Robbins, "our only fault is in subjecting other Cubists' works to the rigors of that limited definition."