In music, an octave is the interval between one musical pitch and another with half or double its frequency. The octave relationship is a natural phenomenon which has been referred to as the "basic miracle of music," the use of which is "common in most musical systems." It may be derived from the harmonic series as the interval between the first and second harmonics. The octave is occasionally referred to as a diapason. The octave above an indicated note is sometimes abbreviated 8va, and the oc… (More on Octaves)