A Key Signature tells us what originating tone a song is to be performed in. There are 12 tones of equal value.

Imagine tones as blocks:

1

2

3

4

5

6

7

8

9

10

11

12

The distance between each tone is called a half-step interval.

Since there are 12 tones in music, there are 12 major Key Signatures in music. 7 of these are primary key signatures called A, B, C, D, E, F, and G.

The remaining 5 key signatures are secondary key signatures. They have dual names, meaning they can be called sharp or flat. A sharp means half a step higher and a flat mean half a step lower. Sharps are represented by a “#” and flats are represented by a “b”. The 5 secondary key signatures are called G# or Ab, A# or Bb, C# or Db, D# or Eb, and F# or Gb.

1	2	3	4	5	6	7	8	9	10	11	12
A	A#/Bb	B	C	C#/Db	D	D#/Eb	E	F	F#/Gb	G	G#/Ab

A key signature can also be a minor key signature which make for a total of 24 key signatures total.

Usually the chord that starts a song will tell you what key a song is being played in. Most songs begin and almost always end with the chord that is the same as the key signature. For example if a song begins with an A major chord, then it’s most likely in the key of A major–but especially if it ends with an A major chord as in the case in the audio example below:

Here is the chord progression that was played: A – D – F#/Gbm – E – A

Exceptions

While most songs begin and end with the chord that is the same as their key signature, there are exceptions. Sometimes a song will start on a chord that is not the same as it’s key signature, but the key signature chord will soon follow it. The chord that is the same as the key signature will usually played more than any other chord. Take a listen to the example below:

This song is in the key of F (F major). Yet it begins with an A#/Bb chord. Here’s the first part of the chord progression: A#/Bb – C – F – A#/Bb – C – F. Clearly it ends with an F chord, and usually this is a dead giveaway. But the bridge that comes next leaves no doubt: Am – Dm – Am – Dm – A#/Bb – C – F. It also ends with and F chord. The lesson learned here is that the beginning may be a little vague, but the ending of a song or chord progression in a song will usually leave no doubt as to what the key signature is.

If a song doesn’t end with the chord that is the same as it’s key signature it usually leaves a feeling of “hanging”. It’s like the song isn’t complete without the key signature chord at the end.

There are songs that end on a chord other than the key signature chord. You can usually spot them a mile away. Some artists use them to create that incomplete or hanging feeling at the end of a song.

You’ve Got To Train Your Ears

You best bet for learning how to determine a song’s key signature is to practice. In fact, it’s the only way you’ll every truly know. Your ears have to be trained to know what to listen for.

Try listening to the radio with your guitar in hand and try to figure out the key signature for each song that plays. First listen for a major or minor key and then try to determine the name of it’s key.