Book excerpt: In much writing on language change, there is a tacit assumption that change operates on a single source construction to produce an innovative target construction. This volume challenges this assumption, by showing that many changes involve interactions between multiple source constructions. In fact, the involvement of multiple source constructions is unexceptional. The phenomenon is observed in phonology, morphology, syntax and semantics. It is seen in language-internal change as well as in contact-induced change. Interactions may obtain between independent but historically related constructions as well as between historically unrelated constructions. The contributions to this volume, on the one hand, present specific case studies on changes involving multiple source constructions, in various domains of grammar and in a variety of languages. On the other hand, they discuss how such changes can be accommodated in current theoretical models of language. Originally published in Studies in Language Vol. 37:3 (2013).