Before explaining key signatures, I suppose we’ll have to discuss what a key actually is. Key signatures sprang up in what Musicologists call the “Common Practice” period of music. This era of musical development spanned from the Baroque period to the end of the Romantic period (roughly, though the dates are negotiable, from 1650-1900). Key signatures came into use to fill the need for greater organization in music. As music grew to become more complicated, equal temperament was developed, and theorists firmed up the rules of harmony, there arose the need for a organizational system to help players read the music in front of them. Prior to the advent of the key signature, during the Medieval and Renaissance periods, there was a great amount of polyphony, with no regard for key.
Each of the 12 notes has a major and minor key associated with it (two concepts we will get to later). Key signatures serve to let the person reading the music which key the music is in and which notes will always be either flatted of sharped.
This is the key signature for Eb major/ C minor. The three flats in front of the clef tell us so. To tell the root key of a flat key signature, look one flat back from the last flat- in this case, Eb. When reading music, it is important to look at the key signature first. the key signature lays out for you the notes which will always be flatted or sharped (unless marked with a natural ♮sign). In this way, key signatures present a short had way to write out music, so that composers will not have to constantly fiddle with writing sharps and flats, and so that performers will not have to read copious sharps and flats.
This is a sharp key signature. Specifically, it is the kay signature for A Major. To tell the root key of a sharp key, look at the last sharp and go up one half step. In this case, the sharp is G#, up one half step is A.
There is, however, a more technical way to learn your key signatures- the Circle of 4ths and the Circle of 5ths.
There is a simple pattern to each block of flat and sharp key signatures:
Let’s break this chart down piece by piece.
You’ll notice, if you move from C to the left, the intervals are increasing by perfect 4ths. This move to the left represents the flat key signatures. You’ll also notice that the order of flats also moves in perfect 4ths, starting with Bb.
Moving to the right denotes the order of sharp keys. The order of sharps in the key signatures move by perfect 5ths, starting with F#.
The numbers in the gray circle represent how many sharps or flats each key signature includes.
The inner green circle represents the corresponding minor keys to the major keys. Minor keys are always one minor third down from the corresponding major key, and, as is shown in the chart, share the same key signature.
I know this is quite a bit of material to wrap your head around, but luckily, music is cumulative, and this concept will keep coming up, over and over again.