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Ok hear me out.

I see some songs written in the key of C and some in Ab and some in Bb and so on and so forth.

But why not use C for all? Since no pitch alone has any inherent quality on their own, unless you have perfect pitch, which most people don't.

We hear music based on the relationships between the notes, so if I play a C major scale to someone randomly, they won't know that it's C major, they can tell it's major but not C major since they don't have perfect pitch.

So if I play "Shape of you" or whatever song in another key to someone, they won't know the key change, unless they have memorized the song or have perfect pitch.

So what's the point of choosing a key? Since it's the relationship of the notes that matter to most people, hence choosing major or minor has value.

  • Why should we limit ourselves? I'll send the question back to you: why should all songs be written in C? – Federico klez Culloca Jan 7 at 7:45
  • O.k. So what's so special about key C? Could this question concern any favourite key - or is C the one for a special reason? Have you considered transposing instruments? – Tim Jan 7 at 9:12
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    'Using different scales?' There are various different scales. This header would make more sense using 'keys' instead of 'scales'. – Tim Jan 7 at 11:28
  • Tim is correct - please amend title if possible. – danmcb Jan 7 at 12:02
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    Your title implies the meaning "different scales/keys in one piece of music" but the detail you add seems to be about transposing a song. Those are two different matters. Are you asking why a particular key is chosen for a song? – Michael Curtis Jan 7 at 17:32
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I see some songs written in the key of C and some in Ab and some in Bb and so on and so forth. But why not use C for all?

The most obvious reasons to choose a different root note for the key are to get the piece into a range where:

  • it sounds good sung by a certain voice
  • it sounds good played on a certain instrument ('no pitch alone has any inherent quality on its own' arguably isn't true when you start considering particular instruments, because an instrument's timbre varies with pitch)
  • it's easy to play on certain instrument (an obvious example is the way that some songs played on guitar take advantage of the open strings, which is more likely to be possible in some keys than others).

Beyond that, your assumption that the root of the key doesn't matter is somewhat valid. Many songs will be written without the composer actively 'choosing' a key - the song will be in the key that happened to fall under the composer's fingers when writing it. Most songs could easily be transposed and still 'work' - in fact they often are, to suit different singers.

However, bear in mind that many musical pieces aren't clearly in one particular key, or change keys a number of times. So having all pieces using the same 'root note' wouldn't really be possible anyway.

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As far as songs that are sung are concerned, it's not the key that takes priority, it's the range and sometimes the tessitura. There will always be a problem with songs that have a big range - the difference between the lowest and higest notes. For some singers, the highest note is just too high to sing well. At that point, the highest note needs to be lowered.

That means every single note in that song also needs to be lowered, by the same amount, obviously. That's where the key needs to be changed.

True, some songs with a small range could successfully be sung by just about everyone in key C. But there are many other songs that, when sung in key C would be too high or too low to be sung comfortably. The same reasoning can also be used with regard to instrument playing. Some instruments have a range which would make playing certain tunes impossible to reach in some keys (including C!).

In any case, what's so special about key C that might give it priority over any of the eleven other keys?!

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...because the contrasting of key/tonalities has expressive effect!.

Playing something like theme #1 in C major and then theme #2 in A flat major is not at all the same as playing both themes in the same key.

The relative relations of tones do define a theme on a self contained level. Meaning on theme from many can be isolated an appreciated from its internal, relative relationships. But that is not the only way the theme gets its expressive force. It also gains expression by its relationship to the key structure of the whole composition.

Actually this effect of themes and the structural relationship of keys is exactly what make sonata form work. When a theme is recapitulated in a sonata its restatement in a different key (typically a sonata theme played in the key of the dominant is recapitulated in the tonic key) the change of tonal center is felt and is the basis for perceiving sonata form. If the contrasting of keys did not have any musical effect, we would not be able to even perceive sonata form!

  • This is definitely important but I think it comes in to play much less for modern music, where songs usually exist within their own microcosm. It's kind of too bad that this is the case. I think it would be great if rock and pop were a little more concerned with the key choices and how they may interact from song to song on an album or in a live performance. I've tried to consider that for albums I've produced but it's not always key that is being considered and songwriters often stick to specific keys. – Basstickler Jan 7 at 17:52
  • My brother's favorite example is Hopelessly Devoted To You from the movie Grease. It contrasts A major and F major. On the other hand, you won't find anything like that going on in most AC/DC songs :-) – Michael Curtis Jan 7 at 18:47
  • I guess I'm drawing a distinction between modulations and keys throughout an album (as a parallel to keys throughout a multi-movement piece), even though your answer didn't draw such a distinction. You can definitely find modulations through rock and pop, not very frequently though. I've been playing in a Whitney Houston tribute band for a couple years (annual local shows) and modulations are huge in her repertoire. – Basstickler Jan 7 at 22:31
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One of the biggest reasons that songs occur in different keys has to do with the instrument playing them or the instrument they were written on.

One very important one is the voice. Singers generally have a specific range that is strong for them and they will sound best singing in that range. Depending on the type of singer, high notes are important for creating the climax of the song (think Whitney Houston or Mariah Carey). We can also keep in mind the ranges that singers have where they transition between chest voice and head voice/falsetto.

Other instruments also have their specifics to keep in mind. The standard low note on a guitar is E, so many songs are written with that in mind. Stevie Wonder is blind, so many of his songs are in keys with lots of sharps/flats, as the black notes on the piano are easy to find, but when bands that I've played in cover Superstition, particularly bands with guitar and no keys, it is almost always played in E instead of Eb. This allows the guitar player to hit the riff with the lowest note in their register instead of having to shift the whole thing up an octave.

Songs that have lots of horns often are written in keys with lots of flats, as most common horns transpose in the flat direction, so it's significantly easier to play songs with lots of flats than lots of sharps.

Beyond the ease of play or the high/low end of instruments, there are also difference in tone when you play in different keys on different instruments. The different ranges of an instrument can create a sort of tension or relaxation as a result of the physics behind the instrument. A stringed instrument will often sound more restricted in higher registers, as the strings have a shorter length to vibrate in. Horns and woodwinds often sound much warmer in their lower registers and more sharp or shrill in higher registers. Playing in different keys necessitates that higher registers be used on some instruments, so this effect will take place frequently for many instruments, regardless of the music the composer is creating.

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