The two closely related Celtic languages Breton and Welsh represent an interesting comparative laboratory for exploring language contact phenomena, since much of the period of their divergence from a common ancestor has been accompanied by intense language contact: Breton with French, and Welsh with English. A number of the ways that the modern forms of the two Celtic languages differ from each other, including reflexive and reciprocal constructions and the encoding of motion events, can be seen as cases in which the two languages have, over time, moved in the direction of aligning their patterns with those of French and English respectively, in a process of convergent evolution which is distinct from, though related to, grammatical borrowing. In neither modern language do the patterns match those of the contact language perfectly, but the isomorphy with French and English is nonetheless striking, especially when seen against the background of other effects of intense contact in these languages.