In music, a fifteenth (sometimes abbreviated 15ma)'s the interval between one musical note and another with one-quarter or quadruple the frequency. It corresponds to two octaves. It's the fourth harmonic. It's referred to as a fifteenth because, in the diatonic scale, there're 15 notes between them if one counts both ends (as's customary). Two octaves (based on the Italian word for eighth) don't make a sixteenth, but a fifteenth. In other contexts, the term two octaves's likely to be used. For… (More on Fifteenth)