Not the most fun topic in the world, I know... but it's something every musician or aspiring musician should thoroughly understand. I've broken down some easy ways you can apply what you already know (but maybe don't realize you do) about key signatures to a new piece of music.
First, we must understand what a key signature is, and how to recognize what all those symbols at the beginning of a staff mean. That's where the circle of fifths comes into play. By studying and memorizing the circle of fifths, you'll be able to look at any piece of music and immediately know that those three sharps at the beginning of the staff mean you're about to play something in the key of A Major - without hesitation!
I know what you're thinking... how in the world am I supposed to decipher this let alone memorize it?? Well, let me help you out. If we start at the very top of the circle of fifths (C Major) and continue clockwise, it's actually pretty simple to unravel. C has no sharps or flats, G has one sharp, D has two, A has three... and so on. And how do we know the order? How did we get from C to G? Again, a pretty simple method - each of the keys are arranged a fifth apart - meaning there are five steps in between each note on the circle (ex. C-D-E-F-G or 1-2-3-4-5). Have you had that "aha" moment yet? Let's all say it together: That's why we call it the circle of fifths! Yes, yes it is.
Alright pupils, now that we've unwrapped the major portion of the circle of fifths, you might notice those lowercase letters on the inside of the circle. If you've had about all you can handle for now with the major keys, you're more than welcome to stop here. If you're feeling ambitious and would like to dig a little deeper, however, let's go! All of the lowercase letters on the circle of fifths represent our minor keys. So, each of these letters is positioned next to a major key, telling us that it is the relative minor of that major key. What do I mean by this? Simply put, these two keys, despite one being major and one being minor, share the same key signature. Take that in for a moment. If we're looking at a key signature in music that has no sharps or flats, we could automatically assume that it's in the key of C major. However, we now know that it could also be in the key of A minor. But how can we know for sure? There are a couple of methods for this. One, if you have a good aural sense of major and minor keys and feel confident in distinguishing the two by sound (some people will say major keys sound "happy" while minor keys sound "sad"), then use that to your advantage! However, if you need a back-up method of knowing for sure, typically a piece will either begin or end on the tonic, or, the note of the key signature. So if you're looking at a piece in the key of C major but could also just as easily be in A minor, check to see if either the first or last note (possibly both) is a C or an A. This just might give you your answer.
So there's a bit about how to know what key signatures mean, and how to identify them. But how can you apply what you already know to a new piece of music? This one is even simpler: scales. If you are familiar with the G major scale, then you have the ability to use that same finger pattern for a piece in the key of G major. How about that? Similarly, with the key of C major, as long as you know that there are no sharps or flats in the key/scale, you will be able to pull that off in a new piece of music. Just remember to not play any sharps (or flats) where they don't belong, and you'll be all set!
Whew! Lot's of information, I know. Hopefully this gave you some insight into the true meaning of key signatures and how you can apply what you already know from scales to your new piece of music! For more information on this topic, be sure to check out my video below!