Schwitters and van Doesburg, Lissitzky presented the idea of an international artistic movement under the guidelines of constructivism while also working with Kurt Schwitters on the issue Nasci (Nature) of the periodical Merz, and continuing to illustrate children's books. The year after the publication of his first Proun series in Moscow in 1921, Schwitters introduced Lissitzky to the Hanover gallery kestnergesellschaft, where he held his first solo exhibition. The second Proun series, printed in Hanover in 1923, was a success, utilizing new printing techniques. Later on, he met Sophie Küppers, who was the widow of Paul Küppers, an art director of the kestnergesellschaft at which Lissitzky was showing, and whom he would marry in 1927. Horizontal skyscrapers In 1923–1925, Lissitzky proposed and developed the idea of horizontal skyscrapers (Wolkenbügel, "cloud-hangers", "sky-hangers" or "sky-hooks"). A series of eight such structures was intended to mark the major intersections of the Boulevard Ring in Moscow. Each Wolkenbügel was a flat three-story, 180-meter-wide L-shaped slab raised 50 meters above street level. It rested on three pylons (10×16×50 meters each), placed on three different street corners. One pylon extended underground, doubling as the staircase into a proposed subway station; two others provided shelter for ground-level tram stations. Lissitzky argued that as long as humans cannot fly, moving horizontally is natural and moving vertically is not. Thus, where there is not sufficient land for construction, a new plane created in the air at medium altitude should be preferred to an American-style tower. These buildings, according to Lissitzky, also provided superior insulation and ventilation for their inhabitants. Lissitzky, aware of severe mismatch between his ideas and the existing urban landscape, experimented with different configurations of the horizontal surface and height-to-width ratios so that the structure appeared balanced visually ("spatial balance is in the contrast of vertical and horizontal tensions"). The raised platform was shaped in a way that each of its four facets looked distinctly different. Each tower faced the Kremlin with the same facet, providing a pointing arrow to pedestrians on the streets. All eight buildings were planned identically, so Lissitzky proposed color-coding them for easier orientation. After some time of creating "paper architecture" projects such as the Wolkenbügels he was hired to design an actual building in Moscow. Located at 17, 1st Samotechny Lane, it is Lissitzky's sole tangible work of architecture. It was commissioned in 1932 by Ogonyok magazine to be used as a