After solving problems revolving around vertical planes, the development of Kupka's painting consistently turnedtowards space. Verticals gave him stability, but he also needed to express tension, which was accomplished with diagonals.
With this came a whole series of pictures called 'Vertical and Diagonal Planes'. Kupka gave the most beautifuland the most perfect of these the title 'Cathedral'. Here he combined his interests in architecture and music, which hehad been interested in since his early youth. It was in France where he became inspired with the architecture of Gothiccathedrals, especially with what he called in his book 'their vertiginous musicality'. The more rhythm there is in a workof art, the closer it approximates music, the art of sounds... Architecture, whose symmetrical alternation correspondsmost closely to musical beats, is according to the situation a hymn, sonata, gavotte, or symphony…' (Tvořeni v uměnivytvarnem/Creation dans l'Art plastique, p. 151).
Kupka also describes the coloured windows that probably inspired this picture: '…as far as the colour purple is concerned,it is necessary to take into account the oscillating speed of red with regard to the speed of blue. In the church ofSaint Germain-L'Auxerrois, on three windows behind the main altar there are meandering borders of blue and red offairly equal area. Up close the blue was dominant. But from a distance, these borders were not purple, as one mightthink they would be, but they would seem red. The blue gets lost on the way…'