In his Vite of 1730 Lione Pascoli reports the opinion of Bernini enthusiasts that Borromini, ‘out of an excessive desire to innovate in ornamentation, had on occasion gone beyond the rules’. Reading this today I think we wonder why they said only ‘on occasion’: how could theorists retain throughout the seventeenth century a conception of architectural rules sufficiently flexible for Borromini ever to be said to satisfy it? This article will examine a small and sometimes overlooked area of architectural decoration — door and window surrounds — and try to discover what such rules might be; firstly by examining the relation between architectural treatises and actual buildings, and then asking to what extent the middle ground between theory and practice can be filled in with a sort of ‘common law’ of architecture: a mixture of precedent, interpretation, and deduction from first principles. It will then attempt not so much to defend Borromini from the charge of going beyond this common law, rather to demonstrate that his innovation depends upon a subtle manipulation of it. In this way I hope to sketch out the dozen or so most important architectural first principles or ‘issues’ which generate virtually every Roman Baroque surround.