Why are there twelve notes in an octave?

Question

I know that one scale consists of 12 half-tones. But my question is still: Why? Why not 13 or 11?

Answer 1

This requires an excursion into musical history.

Originally, instruments were made to simply play notes that sounded "right" together. Why some notes sounded right and others wrong wasn't a great concern for most of humanities history, until Pythagoras, (yes, the guy with the theorem) noticed that it had to do with intervals, and made a music theory based on perfect fifths. This theory had it's problem and was improved upon by later people, eventually ending up on what is called a "just intonation"

Basically, notes sound harmonious if the frequency of the notes is close to a simple interval, like 3/2 or 5/4.

These theories were important because it meant it was possible for different instrument names to make instruments that could play scales together, thereby making orchestras. But just tuning has a problem, you can basically only play the scale the instrument is built for, because the intervals between the notes are different. If you play a tune on the wrong scale, it will sounds out of tune. This means that if you want to sing along with the instrument, you have to find a singer whose range fits the song in the scale the instrument is built for. You can't transpose the song to fit the singer. Also, musicians were exploring the limits of what you could do with just intoned instruments.

So out of this came then the equal temperament. It splits the scale into equal intervals, meaning you can transpose a tune into other keys, and also means you can do dramatic chord changes and other interesting things. You can indeed split the octave into 11 or 13 notes if you should wish to do so, but to most people it will sound out of tune. But when you split it into 12 notes, you get close enough to the seven notes of just intonation for it to be bearable except to some unlucky few supposedly burdened with overactive perfect pitch. The five tones that are in between the basic seven are, as expected, called "half-tones".

There are other equal temperaments than the 12 tones per octave that will sound fine, but they don't generally have a integral number of notes per octave. Wendy Carlos experimented a lot with this, and made such scales as the Gamma scale with a slightly mindboggling 34.29 notes per octave. :-)

Answer 2

This question on math.se is quite similar to what you're asking and the answers give a lot of detail:

Mathematical difference between white and black notes in a piano?

What's going on here is a massively convenient mathematical coincidence: several of the powers of 2^(1/12) happen to be good approximations to ratios of small integers, and there are enough of these to play Western music.

Answer 3

Two points that may have not been completely answered.

• Why is C major the reference scale for natural tones ?

  The anglo-saxon notation obscures the history a little. Tradition from church music led in Italy (then shortly after France and Spain) to naming notes of the reference major scale by conventional syllables: Ut Re Mi Fa Sol La Si (this corresponds to C D E F G A B) coming from the latin lyrics of a very well known piece of that time. The latter single-letter notation takes another starting point, but the reference character of the C major scale has persisted across Occidental countries even if you can find evidence of notations and keyboards using other notes as reference. One of the main influences has been the construction of keyboard instruments (notably the church organ). The current keyboard layout is a compromise between the typical width of the hands, playing the Ut (now mostly called Do or C) major scale easily and having access to all semitones and a few other things. Other designs have not been as successful.

  You also have to know that the theorization and standardization of music at least up to the 19th century was made under the patronage of the churches (orthodox, catholic, reformed, ...) pushing for uniformity. The nineteenth century has seen an even larger standardization and internationalization of tuning, music teaching and piano domination as reference and composition instrument. The last three centuries have progressively suppressed or put into oblivion most of the divergent traditions (as to scales, modes, tuning) in Europe. Nowadays, people learning about music are taught as an evidence the C major scale as a foundation of music theory and the minor scale and his variants is not always treated fairly.

• Why is there a semitone between E & F and B & C and not elsewhere ?

  There are several scales/modes outside of the major scale, with a varying number of notes, where the semitones are not placed between the 3rd and 4th note and between the 7th and 8th. The three minor scales (harmonic, ascending, descending) for instance, but also dorian, phrygian, you can read an encyclopedia article about them.

Answer 4

It has to do with harmony. Notes clash the least when their frequencies match up. For example, a note and its octave match up every two cycles, or a 2/1 ratio. Other ratios that sound good are 3/2, 4/3, 5/3, 5/4, 6/5, and 8/5; these are called the basic consonant intervals. Intervals that clash are the dissonant intervals.

So why twelve notes?

The twelve-tone equal-tempered scale is the smallest equal-tempered scale that contains all seven of the basic consonant intervals to a good approximation — within one percent — and contains more consonant intervals than dissonant intervals.

This page (from which I quoted) provides greater detail: http://thinkzone.wlonk.com/Music/12Tone.htm