Piet Mondrian was a Dutch painter who was a pioneer of abstract art. He was a leading member of the De Stijl art movement, which advocated pure abstraction using only vertical and horizontal lines and the primary colors red, yellow and blue. Mondrian's style evolved from representational landscapes to fully abstract compositions consisting of thick black lines that delineated rectangles of pure color. This style exemplified his Neo-Plasticist theory that art should express universal harmony through basic forms and colors.