A classical church portico – say, St Paul's in London – implies a rite of passage and mediation between the city and the interior rituals of the institution; it beckons the street, while offering shelter to those who want to observe the crowd. In contrast, the spectacular interiors of Modernist churches are often entered without theatre – the plain outer door [1], for example, of Jørn Utzon's Bagsværd Church (Copenhagen, 1973), provides little hint of the magical cloud-like movement of vaulted space within. This is not simply a matter of the changing formal language of architecture (namely, interior space taking priority over exterior shell), but also how the church might be defined in wider civic and historical contexts. Here the Classical theory of decorum remains helpful, for Vitruvius proposed that identifying a building's theme (statione) – that is, the social and institutional destination of the building – was the first step in determining appropriate form, the prerequisite one before considering the related determinates of decor, namely stylistic coherence (consuetudine) and site (natura). The issue of thematic appropriateness is above all important for the front, which announces the building like a frontispiece does a text. Yet as the church portico demonstrates, a thematic motif was rarely the straightforward application of a code, for in the period when architectural decorum prevailed – loosely speaking the fifteenth to nineteenth century in Italy, France and England among other places – how a genre was defined and what constituted the repertoire of appropriate form were contested.