A Dictionary of Music and Musicians/Fifth
FIFTH. A Fifth is the perfect consonance, the ratio of the vibrational numbers of the limiting sounds of which is 2:3. It is called fifth because 5 diatonic notes are passed through in arriving from one extreme of the interval to the other, whence the Greeks called it δια πέντε, Diapente. The interval consists of 3 whole tones and a semitone.
[ C. H. H. P. ]