This is an interesting question because we typically don’t have the opportunity to get in the mind of the composer as they prepare to write a piece. You might wonder if there is something to their method and what is involved in the process.
If you go back in time, tuning was not standardized. Even in Bach’s time, although he wrote pieces in all major and minor keys, they didn’t all sound the same! On keyboard instruments at the time there was no tuning technology invented yet which established a true equal temperament (which is standard today) where all intervals sound the same in all keys.
So, with equal temperament tuning in which all intervals are equal, how do composers pick which keys to write in? Is it something as simple as flipping a coin?
A key that a piece is played in has to fit in the right register. For example, if you were writing a piece for a soprano singer, you wouldn’t want to write too high or too low – you would need it to be in their range so it is comfortable for them. The same thing is true if you are writing for any instrument. For example, the tone of a flute is dramatically different in the low register from the high register.
Going beyond the key of the piece, the octave in which you write in is very important for the sound and mood you are trying to create. There will be times where you are writing something in a particular key and you might find that either octave is not right. You may discover that it needs to fall somewhere between. Therefore, the piece must be written in a different key. These are the things a composer will take into account when writing music.
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