Although Old English [f] and [v] are represented unambiguously in Older Scots orthography by ⟨f⟩ and ⟨v⟩ (or ⟨u⟩) in initial and morpheme-internal position, in morpheme-final position ⟨f⟩ and ⟨v⟩/⟨u⟩ appear to be used interchangeably for both of these Old English sounds. As a result, there is often a mismatch between the spellings and the etymologically expected consonant. This paper explores these spellings using a substantial database of Older Scots texts which have been grapho-phonologically parsed as part of the From Inglis to Scots (FITS) project. Three explanations are explored for this apparent mismatch: (1) it was a spelling-only change; (2) there was a near merger of /f/ and /v/ in Older Scots; (3) final [v] devoiced in (pre-)Older Scots but this has subsequently been reversed. A close analysis of the data suggests that the Old English phonotactic constraint against final voiced fricatives survived into the pre-Literary Scots period, leading to automatic devoicing of any fricative that appeared in word-final position (a version of hypothesis 3), and this, interacting with final schwa loss, gave rise to the complex patterns of variation we see in the Older Scots data. Thus, this devoicing of [v] in final position was not just a phonetically natural sound change, but also one driven by a pre-existing phonotactic constraint in the language. This paper provides evidence for the active role of phonotactic constraints in the development of sound changes, suggesting that phonotactic constraints are not necessarily at the mercy of the changes which conflict with them, but can be involved in the direction of sound change themselves.