Why do octaves start on C not A?

Answer 1

They don't. Octaves can start anywhere.

Although scales are octaves, the standard official octave or octave per se does start at C. You would think it would start at A. Why this is is because of a mnemonic. Musician and music educator Guido d'Arezzo in the 10th century (who probably also invented the musical staff) made a mnemonic based on the 1st stroph of the Hymn to St. John by Paulus Diaconus (Paul the Deacon) which started with the Latin word Ut, which means "that", and was the C note, which became the name for it and was later renamed Do. (However, it might be that the origin is Arabic, from the Darar Mussafalat ("Separated Pearls") which goes dal, ra, mim, fa, sad, lam, ta). The D'Arezzo solmization as it is sometimes called had only 6 notes and the si was added in the 18th century, but the ti is often used instead. The full octave was the now familiar ut re mi fa sol la si ut, except for the ut now being do. Previous to d'Arezzo the notes were designated A-G, A corresponding to la, B to si, etc, this system is also still used today. It is also because the Ionian mode has become our major scale, which starts at C, and has only natural notes (the Aeolian mode has become our minor scale, which starts at A, and has only natural notes, too). As well, the lowest possible audible frequency (16 Hz) is C (C0 in scientific pitch notation). However, there is no definite exact lowest audible frequency as it is often stated to be 20 Hz (which would be at E0) and sometimes 15 (which would be at B0). The maximum audible frequency is usually stated as 20,000 Hz but sometimes at 18,000 so the range is about 10 octaves. This is a great question and has been asked before on Yahoo Answers but with the wrong answers in those cases.