Although this topic has most likely been covered by other users, I decided to write about it myself ---because of a couple of comments I received after an article called Awkward Key Signatures Made Easy'. It was clear from the comments, that the people who didn't understand the point of the article, were a little confused about key signatures in general. Believing that they have more to do with pitch and fingerings than note names. It was this false assumption that was at the heart of the misunderstanding. So I thought I'd go into a little more detail about what a key signature is, which key signatures are most commonly used and why.
What Is A Key Signature
A key signature is nothing more than the marks made at the start of a piece of music - or before a key change - that tell the musician which key the music is in. For example, in the key of G, a sharp sign would be written on the F line. It tells the musician that, unless otherwise indicated, all F's are to be played sharp. This saves the composer having to write a # sign before every F on the score.
How To Write A Key Signature Correctly
A key contains seven notes. When writing the key signature, each of these seven scale degrees must have it's own line on the staff. That is to say, each note has it's own letter name, I.e, A, B, C, D, E, F and G. And each of these must be given a value of either flat (b), sharp (#), or natural (the line is left blank). This means that you can't have two notes with the same name in one key signature. For instance A and A#, or Bb and B. The reason for this becomes obvious when you consider how one goes about leaving a line blank and writing a sharp or flat sign on it as well. This rule can introduce some confusing note names such as: Cb and E#.
The Twelve Most Commonly Used Key Signatures
There'll be some amongst you (of the type I encountered after my last article) who'll be dying to point out that The Twelve Most Commonly Used Key Signatures' is a bit of an oxymoron. But wo there horsey. Theoretically, the number of key signatures is infinite. Remember that they're just names for things. Just as Bb and A# are 2 different names for the same thing. When dealing with key signatures, It's better not to think in terms of pitch and fingerings. Again, I'd like to emphasise that this is about written music and not the fret or keyboard patterns.
So why is it preferable to mark the key as Ab rather than G#? Well, let's use the circle of fifths to work our way through and find out.
At this point I'm going to substitute F# for Gb and continue moving up in fifths.
KEYNo. of sharps
D2 (F, C)
A3 (F, C, G)
E4 (F, C, G, D)
B5 (F, C, G, D, A)
F#6 (F, C, G, D, A, E)
So we began with C containing no sharps, and we moved through the key signatures in such a way as to arrive back at C, which also contains no flats. A perfect circle. And isn't it a beautiful thing? Look at the symmetry. Not only do we move up in fifths to discover that the next key contains one more sharp, but we also move up in fifths to find out which sharp it is. F up to C, up to G etc. Adversely, if we start at C containing no flats and move DOWN in fifths, not only do we discover that the next key contains one more flat, but by moving down in fifths we also discover which flat it is. B down to E, Down to A etc. Wonderful!
So now we can see why we write in Bb and not A#. If, instead of substituting F# for Gb, we were to continue on with the first table, we'd find that C# was the next key. And using the formula established by the previous keys we'd also find that it contains 7 sharps, and they are: F, C, G, D, A, E, B. So now as well as an E#, we now have a B#. Double trouble! But we talked about A# so we have to journey further up the ladder of insanity. The next key would be G#, which according to our formula must contain 8 sharps. But there are only 7 scale degrees, and we sharpened all 7 for C#. And if we look at the fifths: F, C, G, D, A, E, B, the next one is F again. So what's going on. The fact is that the seventh scale degree is always a semitone below the root. So in the case of G#, this means that F's are double sharp ##. Hey Presto! 8 sharps! You can continue alone if you want to move through D# and onto A# if you like. I'm bailing out here. Hopefully that cleared a few things up.
KEYNo of flats
Gb6 (B, E, A, D, G, C)
Db5 (B, E, A, D, G)
Ab4 (B, E, A, D)
Eb3 (B, E, A)
Bb2 (B, E)