I was working with a student recently and got into the discussion of key signatures. The student was a beginner and had heard of key signature but didn’t really understand anything about them.
So to help them out I told them this:
You can think of key signatures as scales. In a way they are the same thing. By this I mean when we say that a song is in the key of “C Major” then the song is based on the C Major scale.
If you have heard or seen my lesson on Harmonic Structure (it’s in the paid version of Rock Guitar Power), you’ll understand that a major scale has 7 notes in it and on every note we can build a chord. Therefore there are 7 chords that go along with any key (you can build different types of chords such as Maj 7th and Min 7th but to keep this simple we’ll just work with regular major and minor chords).
I then showed her an example of a song that is a 12 bar blues in the key of “C”. I first showed her how the C Major scale has these notes: C D E F G A B C. Then I told her that the 12 bar blues progression uses the 1, 4 and 5 chords from the key. So that means that you use a C, F and G chord to play the 12 bar progression.
After describing it to her this way it made sense and she was able to understand what I mean when I say that a song is in a particular “key” and how that is based on a particular scale.
Hopefully this makes sense to you as well. If it doesn’t or you would like me to further explain the theory of key signature, scales and how they relate, please leave a comment and I’ll be happy to dig in more!