There are two ways to identify a key:
- Look at the key signature.
- Analyze the chords of the song.
If you happen to be playing from music written in standard notation, such as a songbook or hymnal, the easiest way to identify the key is to look at the key signature—the number of sharps or flats written at the beginning of the staff between the clef symbol and the time signature.
Identifying a major key from the key signature is actually easy:
- Look for sharps or flats in the key signature.
- If there are no sharps or flats, the key is C. If there is one flat (B flat), the key is F.
- If there is at least one sharp or more than one flat, observe the position of the last sharp or flat. In the example above, the sharp symbol is marking an F sharp.
- If the last note is sharp, count one half-step up to find the key. In the example above, one half-step up from F sharp is G. Therefore, the key is G major.
- If there is more than one flat, the second to last identifies the key. In the example below, the second-to-last note in the key signature is D flat. Therefore, the key is D flat major.
But what if the song is in a minor key? The answer is still simple:
- Identify what the key would be if it were major. For example, if you see a key signature with no sharps or flats, the key is C.
- Count backwards three half-steps to find the name of the minor key. For example, three half-steps down from C is the note A. Therefore, a minor key with no sharps or flats in the key signature is A minor.
If you are playing music by ear or from a piece of tablature, finding the key is even easier. Find out what chord ends the song. The name of the ending chord is almost always the name of the key. If a song ends on a G chord, the key is G major. If a song ends on an A minor chord, the key is A minor.
Different instruments lend themselves well to different keys. Songs in the keys of C and G are generally very easy to play on guitar, while fiddlers typically prefer songs in the keys of A and D. For an extra twist, a harmonica is tuned to one key only, so a harmonica player must have a new instrument for every major key!
In a group setting, instrumentalists must compromise with each other and with the vocalists, who probably have certain keys they are most comfortable with. For this reason, it is best to become comfortable with the scales and chords of many different keys. Making scale and chord studies a regular part of your practice session will pay off, both when you are playing with others and when you just feel like doing something a little different in your arrangements.
Also, depending on what instrument you play, you will definitely want to invest in a capo. This device makes switching keys simple. If you are used to playing a guitar in the key of C, for instance, you can easily play the same song with the same fingering in the key of D by just putting a capo at the second fret (D is two half-steps up from C).