This chapter presents an overview of the conception of semai from the mid-seventeenth to the nineteenth centuries and suggests explanations as to the historical trajectory of the usul's performance practice. It describes the performance practice of semai before the nineteenth century and explains the development of the forms and types of the usul. The historical trajectory of semai may have shared features with that of other usuls. Vocal and instrumental works composed in semai–the only usul directly linked to a specific compositional genre–have been essential parts of the classical fasil repertoire. In the post-seventeenth-century classical Ottoman music tradition there were distinct usuls sharing a common name, such as berefsan and nim/aksak berefsan. Remarkable differences have been observed between pre-nineteenth-century and sources in the description and transcription of semai. In the context of the usul–melody relationship in Ottoman music, a group of pieces written in two different usuls, with different patterns and numbers of beats, is a peculiar situation.