Fifteenth

In music, a fifteenth or double octave, abbreviated 15ma, is the interval between one musical note and another with one-quarter the wavelength or quadruple the frequency.

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) do not make a sixteenth, but a fifteenth.

However, because of the large frequency distance between the notes, it is less likely than an octave to be judged the same pitch by non-musicians.

Either direction can be cancelled with the word loco, but often a dashed line or bracket indicates the extent of the music affected.