There’s an old joke that goes, “How do you get your guitarist to turn down his amp? Answer: Put sheet music in front of him.” Too often, guitarists never become fluent in reading sheet music.
It’s confusing. You look at a piece of sheet music, thinking about maybe trying to play it, and the first thing you see is a bunch of sharps or flats, most of the time. If there aren’t any, you know it’s in the key of C. If there’s just one sharp, you know it’s in G. If there’s just one flat, you know it’s in F, which for most of us isn’t very helpful. That’s about as far as most guitarists get in learning to decode key signatures. You could memorize all twelve, but there’s a much easier way.
Look at the signature, the group of flats or sharps at the very beginning of the sheet music. Reading from left to right, you will see anywhere from zero to six of them, showing which notes are to be sharped or flatted. For example, suppose a piece has a signature of four sharps: E#, C#, F#, and D#. It doesn’t matter how many there are, or which notes they are on. Just look at the last one, (reading from left to right), and raise it a half tone. In this case, you get E. That’s your key. Simple!
The trick for flats is even simpler. Just look at the next-to-the-last flat. If there are four flats: Bb, Eb, Ab, and Db, the key is Ab. All key signatures with flats in them are flatted keys: Ab, Bb, Db, Eb, Gb. The only exception is F, which has one flat. But you already knew that. Few guitarists like to play in any of these keys without using a capo. So, determine the key shown by the next-to-last flat, drop it another half note (in this case to G), and play it with a capo in the first fret. Easy!
These rules are for MAJOR keys. MINOR keys are a bit more complex. Each minor key shares its signature with a major key, and is called its “relative minor.” Am shares its key signature (no sharps or flats) with C. Em is the relative minor of G; Bm is the relative minor of D, etc. As a guitar student, you learned four basic chords in each key: the Dominant, Sub-Dominant, Seventh, and the Relative Minor. Now you know why it’s called the Relative minor.
Here’s are the major keys with their relative minors:
Major Key Relative Minor
See the pattern? Each Relative Minor is exactly three half-notes (frets) below it’s major key. To find a minor key, just find the major key from the key signature, drop three frets, and you’re there!