This article explores the importance of register variation for analyses of grammar and discourse. The general theme is illustrated through consideration of variability in the form and use of English complement clauses. First, the patterns of use for four related grammatical constructions are considered: that-clauses and to-clauses, headed by verbs and by nouns. The differing discourse functions of each construction type are explored by considering their lexico-grammatical associations (i.e. the verbs or nouns most commonly occurring as the head of each type). However, it is shown that the characteristic uses of each type are conditioned by register. That is, each construction type has a different distribution across spoken and written registers, with a different set of associated lexical heads. A second study provides an even more striking illustration of this interaction between grammar, discourse, and register: the contextual factors conditioning the retention vs omission of the complementizer that. In this case, it is shown that each register has an overall norm, and that contextual factors are influential only when they work in opposition to that register norm. These case studies are presented to make the general point that analyses of grammar and discourse are often inadequate and misleading when they disregard register differences. Instead, a register perspective is required to capture the range of variability associated with grammatical patterns of use.