This chapter focuses on phrase structure, whose discussion in the eighteenth century was subsumed under the theory of melody and based on the parallel between music and language. The first part is devoted to classification of caesuras and melodic sections contained by them. Since the former were equivalent to punctuation marks (period, colon, semicolon, comma) and the latter to grammatical units (sentences, clauses), the musical terminology adopted by eighteenth-century authors (Johann Mattheson, Joseph Riepel, Friedrich Wilhelm Marpurg, Johann Philipp Kirnberger, and Heinrich Christoph Koch) was influenced by linguistic terminology and it developed for decades, with meanings of individual terms changing from author to author. The second part of the chapter treats the different lengths of phrases. It links the preference for four-measure phrases to regular hypermeter and it presents a classification of four-measure phrase rhythms.