The late-medieval Scottish-English border had a porous impermeability. Politically, there was in theory a demarcation between Scotland and England that remained relatively fixed from the thirteenth through the sixteenth centuries, though a zone of marches surrounded that demarcation, and border disputes continued throughout the period. On the English-Scottish linguistic border, such porous impermeability took several forms. The extremes would be works written primarily in either Scots or some form of English and arrayed between them is what might be called the interlinguistic marches: works written in a language neither entirely Scots nor entirely English that somehow depends on and elides any easy distinctions between the varieties. These marches, and what they say about the Scots-English linguistic border, are the focus of this paper. The paper begins by looking at several texts whose language challenges any easy pronouncements about Scots-English dialect boundaries. Originally written in one variety, they survive as well in copies wherein the text has been rewritten, in all or in part, in the other variety. These rewritings might be called 'dialect translations', though other terms also could apply, each of them reframing not only the rewritings but also the varieties and language dynamics they help create. From these examples the paper turns to the larger issue that is its primary concern: the medieval English linguistic repertoire, including the grammatical integrity and social significance of medieval English regional varieties in general. Language and dialect boundaries certainly do matter, but it is speakers who decide how and when.