This dissertation seeks to introduce, identify, and document a set of syntactic insertions in sonata forms that I call extrinsic phrases. At its most basic, “extrinsic phrase” refers to any phrase-length insertion between the transition and secondary theme of a sonata-form movement. This concept encompasses several terms (or examples thereof) from current writings about sonata forms. However, unlike those terms, I purposefully define extrinsic phrases broadly so that they act as a generic option for an analyst. Instead of attempting to create a long-range view of extrinsic phrases across the whole of sonata-form writing, this initial work on extrinsic phrases focuses on the early- and mid-eighteenth century. It examines two types of extrinsic phrases—those that change an exposition’s normative tonal trajectory and those that do not—in the works of Luigi Boccherini, Domenico Scarlatti, Joseph Haydn, and their contemporaries. The analyses show that the extrinsic-phrase concept creates several advantages for an analyst. Because they remain flexible in their definition, extrinsic phrases allow one to identify similarly-functioning passages that use disparate phrase and cadence structures. It also allows one to examine a variety of seemingly disparate compositional practices. This has the potential to reveal connections between composers and compositions of an equally wide variety of time periods.