This article deals primarily with a small repertory of early fifteenth-century motets that survive with tenores ad longum - substitute parts for notationally difficult tenors: works by Dunstaple, Binchois, Ciconia, Brassart, Velut, Carmen and Antonius de Civitate. The eight cases, together with Du Fay's O Sancte Sebastiane and the fourteenth-century motet Inter densas, are discussed individually, with consideration of the reason for inclusion of a tenor ad longum or other rhythmic cues, and who might have been responsible for the part.