Pre-R Dentalisation (PreRD), the dental pronunciation of /t/ and /d/ before /r/ and /ər/, is a well-known feature of English varieties throughout Ireland. PreRD is often accompanied by an /r/-Realisation Effect (RRE), whereby /r/ is pronounced as a tap after the dentalised consonant, and a Morpheme Boundary Constraint (MBC), such that PreRD is blocked by Class 2 morpheme boundaries. Although an Irish origin for PreRD has been suggested, the presence of PreRD, the RRE and the MBC in northern English dialects in a form nearly identical to what is found in Ireland suggests that the origins of PreRD lie instead in English in Britain. The possible existence of PreRD in Scotland is suspected, but definitive evidence for PreRD, the RRE and the MBC there has never been published. In this article, I provide the first detailed analysis of these features in Scotland, using unpublished data collected as part of the Linguistic Survey of Scotland. It will be seen that there is substantial evidence for PreRD, the RRE and the MBC in Scots dialects. The presence of these features in Scotland has important consequences for their history in Britain, and confirms the British origin of PreRD in Ireland.