‘En Angleterre plus qu'ailleurs, les catholiques sont obsédés par les minuties des observances liturgiques' wrote Joseph Rykwert in an essay on church art of 1958. Laws set down by the Church prescribed certain forms in Roman Catholic church design in the mid-twentieth century. Liturgical furnishings, artworks and the general concepts of churches were imposed by canon law, the rubrics of the missal, and instructions issued by the Vatican and local hierarchies. In Britain in the 1950s and early 1960s, such laws were considered by clergy as vital to the integrity and distinctiveness of the Church, and as contributing to the efficacy of ritual. Meanwhile a new generation of architects espousing the Modern Movement wished to challenge accepted conventions, questioning the meanings and purposes of every element they were asked to design. In doing so, they confronted a clergy often obsessed with legislation. Sometimes the result was retrenchment, or a compromise: one radical element might be allowed provided a second orthodox one was provided in parallel. But in some interesting cases architects' attempts to reinterpret rules inspired radical new forms. Stations of the cross might be analysed and integrated into architecture; altars and baptisteries could be carefully refashioned for symbolic ends. Many modern architects sought the purposes and spirit behind the rules, testing their limits, sometimes appealing to ancient precedent for a rediscovery of fundamental principles. The period of transition after the Second Vatican Council introduced new levels of complexity, as laws were continually being issued and revised, and the notion of legislated form gave way to more general principles and freer and therefore supposedly more sincere expression in both liturgy and architecture. This opened up further opportunities for architects to rethink church design beyond the limits they had previously been given. In analysing selected case studies through textual rubrics, and with archival sources to hand, this paper uncovers a mode of operation in modern architecture, working in the spaces between and beyond the given rules.