Van der Leck's use of only primary colors in his art greatly influenced Mondrian. After a meeting with Van der Leck in 1916, Mondrian wrote, "My technique which was more or less Cubist, and therefore more or less pictorial, came under the influence of his precise method." With Van Doesburg, Mondrian founded "De Stijl" ("The Style"), a journal of the De Stijl Group, in which he first published essays defining his theory, which he called neoplasticism. Mondrian published ""De Nieuwe Beelding in de schilderkunst"" ("The New Plastic in Painting") in twelve installments during 1917 and 1918. This was his first major attempt to express his artistic theory in writing. Mondrian's best and most-often quoted expression of this theory, however, comes from a letter he wrote to H. P. Bremmer in 1914:  When World War I ended in 1918, Mondrian returned to France, where he would remain until 1938. Immersed in post-war Paris culture of artistic innovation, he flourished and fully embraced the art of pure abstraction for the rest of his life. Mondrian began producing grid-based paintings in late 1919, and in 1920, the style for which he came to be renowned began to appear. In the early paintings of this style, the lines delineating the rectangular forms are relatively thin, and they are gray, not black. The lines also tend to fade as they approach the edge of the painting, rather than stopping abruptly. The forms themselves, smaller and more numerous than in later paintings, are filled with primary colors, black, or gray, and nearly all of them are colored; only a few are left white. During late 1920 and 1921, Mondrian's paintings arrive at what is to casual observers their definitive and mature form. Thick black lines now separate the forms, which are larger and fewer in number, and more of the forms are left white. This was not the culmination of his artistic evolution, however. Although the refinements became subtler, Mondrian's work continued to evolve during his years in Paris. In the 1921 paintings, many, though not all, of the black lines stop short at a seemingly arbitrary distance from the edge of the canvas, although the divisions between the rectangular forms remain intact. Here, too, the rectangular forms remain mostly colored. As the years passed and Mondrian's work evolved further, he began extending all of the lines to the edges of the canvas, and he began to use fewer and fewer colored forms, favoring white instead.