The introduction of the monumental dome chamber during the Saljuq period can be seen as a major achievement in Islamic architecture. The brick constructions are apparently well thought-out and were built with great exactitude. Can we determine some of the guiding principles behind their design? How did the builders balance the request for a lofty dome with the limitations in the span of its vaulting? New data from two mosques of the Saljuq period can be adduced to get closer to possible answers. While the little mosque of Qerve appears rather as a routine construction, the mosque of Borujerd presented a greater challenge.