In Present-Day English clauses containing a subject, a finite auxiliary and a non-finite verb, there is variation with respect to adverb placement in the clause-medial domain: Adverbs may precede or follow the auxiliary (SAdvAuxV vs. SAuxAdvV). However, the latter option is strongly preferred. This paper explores the diachronic development of this variation. It will be shown that the marginal status of SAdvAuxV is a stable feature of the syntax of English throughout its history. The rate of SAdvAuxV order consistently remains in the range of 2% to 7%. This diachronic stability of a low-frequency word order phenomenon raises the question as to why, in contrast to many other cases of linguistic variation, the clear minority option has not been driven out of the grammar over time. We propose that the maintenance of SAdvAuxV order is the result of the presence of contexts that either strongly favour pre-auxiliary placement of adverbs or make it necessary. These include clauses without overt subjects, clauses with adverb scope over reduced negation, and clauses with non-low stress on the auxiliary. In all these contexts, semantic or prosodic factors can be argued to lead language learners to maintain a syntactic option deriving AdvAux order.