In music, a fifteenth (sometimes abbreviated 15ma) is the interval between one musical note and another with one-quarter or quadruple the frequency. It corresponds to two octaves. It is the fourth harmonic. It is referred to as a fifteenth because, in the diatonic scale, there are 15 notes between them if one counts both ends (as is 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 is likely to be used… (More on Fifteenth)