How did Piet Mondrian’s ideas differ from Theo van Doesburg’s later ideas for De Stijl?

The primary divergence between Piet Mondrian and Theo van Doesburg regarding the De Stijl movement centered on the use of diagonal lines.1

While Mondrian’s Neoplasticism strictly restricted pictorial language to horizontal and vertical lines, primary colors, and neutral tones to achieve a sense of universal harmony and spiritual order, van Doesburg later sought to expand the movement's aesthetic. Around 1924, van Doesburg introduced the diagonal into his compositions to create a more dynamic effect, a concept he termed "elementarism".231

Mondrian fundamentally disagreed with this departure from the movement's established principles, viewing the introduction of diagonals as a violation of the pure, balanced abstraction he championed. This ideological conflict was significant enough that it led Mondrian to reject the De Stijl movement entirely. While Mondrian remained committed to his rigid, orthogonal vision, van Doesburg’s later work reflected a broader interest in diverse artistic influences, including Dada, and a desire to evolve the movement's formal constraints.1