Why does a major scale have seven notes?

By Dr. Sean Hutchins

Music history reveals how different cultures use different scales as part of their compositions: traditional Chinese music, for example, is often based on a pentatonic scale, with five notes. Classical Indian music and gamelan music of Indonesia both use distinctive pitches in their music as well. Although these may sound out of tune to a Western listener, to a person who has grown up with these styles of music, it will sound perfectly natural, in much the same way that one’s native language is understandable.

When we look at the physics of sound, an octave, for example, is what you get when one sound is twice as high as another. Because of the way that sound waves combine, sounds in small-number ratios like this (2:1, in an octave) tend to reinforce each other in regular patterns, whereas large-number ratios (31:30, for example, in a quarter-tone) do not. Thus, the former sound consonant, but the latter sound dissonant. Our scales tend to prefer consonant sounds with small-number ratios, such as 2:1 (octave), 3:2 (perfect fifth), and 4:3 (perfect fourth). A chromatic scale divides the octave into 12 equal steps, chosen to have as many small-number ratios as possible while still allowing for music to be transposed to other keys (think Bach’s “Well-Tempered Clavier”).

Most Western music is not based on the chromatic scale, but major or minor scales, which have seven notes taken from this chromatic scale, in a combination of whole and half steps. Cognitive psychology can help explain why this is.

Studies have shown that our short-term memories are quite limited. In one well-known article from 1956, psychologist George Miller described how people can remember lists of numbers or words nearly perfectly when there are five or fewer, but accuracy drops drastically when the number goes past nine items. Likewise, he found that people could remember a sequence of up to seven tones, but not much more. This limit on short-term memory capacity was termed “the magical number seven, plus or minus two”. This may be one reason why we use seven notes in our scales, rather than 12, in order to help us remember more easily.

And why do major and minor scales have some half and some whole steps? Why not a six-step whole tone scale, or an octatonic scale? Having a mix of different types of steps allows us to orient ourselves within the scale. When each note in the scale has a different relation to the others, it allows notes to have individual functions, such as the tonic, or a leading tone. This differentiation of functions also makes the scales easier to remember.

There are cultural, physical, and cognitive factors that go into making our scales what they are, and when you put these together, you get the major and minor scales — simple and easy to remember, but complex enough to support a wide range of musical styles.