For many people, key signatures can be a somewhat daunting prospect. You look at a piece of music with 5 flats on a page and you have no idea what to say. Maybe you have been playing this song by yourself and now you are playing it with a group. They ask you what key it is in, and you cant answer, you dont know.
Well I am here to tell you that there is a second way to do it besides the one mentioned in my previous post on the topic. In fact, Im surprised I did not mention it before. It is known as the circle of fifths. No, I would like to say that using this method in no way means you should not memorize your key signatures, being able to quickly recognize what key a piece is in can be very important. But it does allow you to figure out relatively quickly what key something is in. It is also a good aid for memorization.
The circle of fifths is built on intervals. Due to the way that. Going clockwise around the circle, every note is a perfect fifth above the previous one. C has no flats or sharps. G, on the other hand, has one sharp and is consequently a perfect fifth above C. To continue on, D is exactly a perfect fifth above G and has 2 sharps, and so on. This would continue on to D flat, which has 7 sharps and is a perfect fifth above F# (or G flat).
Now for the flats. The principle works the same way for flats, except in reverse. Instead of going a perfect fifth up, you go a perfect fifth down. An F is a perfect fifth below a C. Or, in other words. A C is a perfect fifth above an F. However, you should not that a perfect fifth down, is the same as a perfect fourth up. For this reason, the circle of fifths is sometimes called the circle of fifths and fourths.
The circle also works, naturally enough, for minor keys. If you know you are in a minor key, but are not sure which, simply start at A and work your way up or down. You will find the key you are looking for soon enough.
This allows you to tell, from any instrument (but especially a piano) what key you are in just by looking at the notes on the instrument. It can be incredibly helpful.
Use this enough and I know that you will get this memorized. Then, before you know it, you will have to reference it less and less. It will start out slow. One or two sharps is easy enough to remember, and eventually more and more key signatures will be added on until you know it by heart.