The name De Stijl is supposedly derived from Gottfried Semper's "Der Stil in den technischen und tektonischen Künsten oder Praktische Ästhetik" (1861–3), which Curl suggests was mistakenly believed to advocate materialism and functionalism. The "plastic vision" of De Stijl artists, also called Neo-Plasticism, saw itself as reaching beyond the changing appearance of natural things to bring an audience into intimate contact with an immutable core of reality, a reality that was not so much a visible fact as an underlying spiritual vision. In general, De Stijl proposed ultimate simplicity and abstraction, both in architecture and painting, by using only straight horizontal and vertical lines and rectangular forms. Furthermore, their formal vocabulary was limited to the primary colours, red, yellow, and blue, and the three primary values, black, white, and grey. The works avoided symmetry and attained aesthetic balance by the use of opposition. This element of the movement embodies the second meaning of "stijl": "a post, jamb or support"; this is best exemplified by the construction of crossing joints, most commonly seen in carpentry.