Key signatures are a great way to know what notes are going to sound good without even playing them. To find out what key a song is in, you must take into account what chords are played and what single notes are played. With these two items, you can figure out what key a song is in. Key signatures were made for musical notation. They are a group of sharps or flats placed at the beginning of the music notation. They are useful because they assign accidentals to notes throughout the whole piece of music without having to write a sharp or flat symbol next to the note every time. The key signature comes into play with guitarists when common scales and patterns are played. A player can simply say “the key is A blues,” and a guitarist instantly knows he can play the A blues scale without worrying.
Circle of Fifths
An easy way to find out the key signature of a piece is to look at the circle of fifths. The circle of fifths is a chart that can be used to find out how many sharps there are in each key. At the beginning of a song (in music notation, NOT TAB) there will be a number of sharps or flats. Each accidental will be on the line of the note that it modifies. If there are no sharps or flats, the key is Cmaj or Amin. Count the accidentals and compare them to the chart. If your music has 1 sharp, it is F major. Make sure if there are sharps, you use the right side of the chart, since the left side is for flats. If there are 3 flats, the key is Eb major. The numbers inside the circle correspond to the number of sharps (right side numbers) and the number of flats (left side numbers). There are a lot of different charts of the circle of fifths. Some include the corresponding minor scale, and some don’t. Try to find one you like, or just use the one in this article.
Keys and Chords
You can also tell what key you're in by studying the chords you're playing. When you’re playing just power chords using the root, 5th and 8th notes, you can judge by the root notes of the chords what key you’re in. For example, using the chords C5, D5, G5 you could be in the key of Gmaj OR Cmaj since both keys contain the notes C, D, and G. To really narrow down the key you’re in, you have to use chords that contain more notes. If you add in the 3rd to each chord it only leaves the key of Gmaj because the third of Dmaj is an F#, which doesn’t exist in the key of Cmaj. This shows why keys are important. If you’re playing these three chords, C, D, and G, you could maybe get away with playing a Cmaj scale for the melody by leaving out some notes, but it would be more to your advantage to use all the notes of the Gmaj scale.