The south tower, the shorter of the two and unfinished, had insufficient foundations and was unstable from the beginning. It was eventually reinforced in 1314 with a massive buttress on its flank. This buttress, besides supporting the tower, contained a stairway and the small prison operated by the Cathedral chapter. The upper room of the buttress was used as the office of the architect, and has plans of the bays and a rose window etched on the stone floor, where they could be consulted by Cathedral builders. In the 18th century, it was used for a time as a studio by the painter François Boucher.