Theo van Doesburg was a Dutch painter, designer, and art theorist. As the founder and major polemicist of the avant-garde movement known as De Stijl (The Style), he was instrumental in developing an abstract style based on primary colors and geometry. Tirelessly promoting De Stijl across Europe, van Doesburg played a crucial role in the development of Modernist art, architecture and design in the first half of the twentieth century. Born Christian Emil Marie Küpper in Utrecht, van Doesburg was the son of the photographer Wilhelm Küpper. His pseudonym was developed from the name of his stepfather, Theodorus Doesburg, whom he regarded as his natural father. Van Doesburg became a painter around 1900. His early work was influenced by Post-impressionism and Fauvism, but in 1915 he discovered the work of Piet Mondrian and underwent a profound transition. Mondrian had developed an austere visual style based on primary colors and orthogonal grids. This convinced van Doesburg to pursue spiritual harmony based on mathematical order.