Why are there only 7 musical notes? Was it decided to divide sounds like that or are there no more in nature?
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There are 7 notes in most musical scales in western music. But there are 12 notes in western music, we just don’t use all 12 notes for every scale.
The reason we have these notes is mostly because of a thing called *[harmonics](https://en.wikipedia.org/wiki/Harmonic_series_(music))*. Harmonics are a natural phenomenon of how waves propagate in various ways.
One way is plucking a taught string, like on a guitar. When you pluck a string, you get a note. We call this the first harmonic, or the fundamental. If you hold your finger down at the point in the exact middle of the string (this means that you’re dividing the length of the string in half), then you get a note that is an octave of the open note. This note is the second harmonic of that string. If you divide the string into a third of the length, you get the 3rd harmonic, which is called a perfect 5th from the open note. Divide it into a 4th of the length, another harmonic. [And so on](https://upload.wikimedia.org/wikipedia/commons/c/c5/Harmonic_partials_on_strings.svg).
The first 27 harmonics of any note essentially lay out the other 11 notes (this isn’t exact, since pitches are not of equal distance to each other and we can only approximate them in western music). But why 7 notes instead of 12?
The first two harmonics are essentially the same note. But the third harmonic is what is called a perfect 5th. We call it that because of a long history of it being the most *consonant* note, other than the octave. Consonance basically means that two notes sound good together. There’s a physiological reason for this, but there is also a mathematical reason for this. Both are a bit complicated.
Since octaves and 5ths sound good to us, it is natural that our ancestors started to build their idea of music from both. Our notes repeat at the octave, but they do something weirder with the 5th. If you start at a note, then move up a 5th, then move up a 5th from that, and keep going until you have 7 notes, then you have just found the 7 notes of a major scale. And this is likely why we have 7 tones in the major scale, which is the basis for most of western music.
##edit: beyond ELI5
there’s a lot more to this that I think is really cool. The harmonics of a note (any note) are based on integer multiples of its frequency. Which means that if the note A has a frequency of 220 Hz, then all of its harmonics will be multiples of 220:
Harmonic | Frequency | Note
—|—|—-
1 | 220 | A3
2 | 440 | A4
3 | 660 | E5
4 | 880 | A5
5 | 1100 | Db6
6 | 1320 | E6
And they continue infinitely, getting harder and harder to hear with each multiple. Other than the fundamental (A), E is the strongest harmonic, and the most pleasing to our ears (since it has a simple ratio from the fundamental). E is the fifth of A.
These harmonics aren’t just created by “dividing the string length”, *they exist in the fundamental itself*. If you were to decompose the sound of a single string pluck the same way you decompose light through a prism into its rainbow components, then you would find some combination of these harmonics (and more, but the harmonics would be strongest).
So in a lot of ways our system of 12 notes with 7 notes in a scale (or key) is based upon the relationship of the 5th from the fundamental. And this relationship is encoded in the very nature of every single individual note. And not only notes, but the harmonic series shows up in other natural phenomenon.
Or course, all of this is ignoring the difference between [just intonation](https://en.wikipedia.org/wiki/Just_intonation) and equal temperament, other tuning systems, as well as the [pythagorean comma](https://en.wikipedia.org/wiki/Pythagorean_comma)).
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