The black saw-wing (Psalidoprocne pristoptera), also known as the blue saw-wing or black rough-winged swallow, is a small passerine bird in the swallow family. The black rough-winged swallow is a small swallow at 13–15 cm length with a forked tail. Its plumage is blue-glossed black. Sexes are similar, but the female has shorter outer tail feathers, and less obvious wing serrations. Juveniles are brown with little gloss, and have short tails. There are many subspecies of this swallow, which some authorities may split into different species. In particular, four northeastern races, including nominate P. p. pristoptera, have conspicuous white underwing coverts (all other subspecies are green-glossed and have completely dark underwings), and may be split as the eastern rough-winged swallow or eastern saw-wing (swallow), P. orientalis. This leaves P. p. holomelaena as the black rough-winged swallow, P. holomelaena. Other subspecies are also sometimes elevated to species status, but Turner and Rose take the view that all the races of the black rough-winged swallow are, at best, incipient species.