Key Signature and the Circle of Fifths: A Complete Guide

When it comes to basic concepts in music, few are as important as key signature. Without a fundamental understanding of how key signature functions, you won’t be able to play music correctly.

Key signature may seem complex; at first glance it seems like a random assortment of sharps or flats with no obvious rhyme or reason.

Rest assured, however, that key signature follows a very strict set of rules. In this guide we’ll look at how key signature works in great depth, and see how it relates to the circle of fifths.

What is Key Signature?

Key signature
The key signature (in this case A Major) is circled in blue.

Key signature is the set of sharps or flats (known as accidentals) that you see at the beginning of a piece of music. The key signature has two primary functions:

  • It helps you find out what key a song is written in; with the key signature you can narrow the choices down to two keys: a major and its relative minor. For instance, if the key signature shows no sharps or flats, you’ll know that it’s written in either C Major or A minor. We’ll cover this in-depth later.
  • Key signature tells you which sharps and flats you’re supposed to play, without having to write one for every single note. For instance, if an A# is in the key signature, this tells you that you should always play an A# where you see an A (unless canceled by a natural sign).

These two concepts are really one and the same. If you know what sharps and flats to play you’ll know what key you’re playing in, and vice versa.

Why does key signature matter?

Knowing what key a song is also helpful because it helps you understand what the tonic, or “root note,” is. Songs are built around a tonic, and knowing what note that is will help you understand the mechanics of the song on a deeper level.

Here’s an example: If a song is in C Major, it almost always resolves (or comes back to) a C Major chord at some point.

Leonard Cohen’s Hallelujah (written in the key of C Major) is a great example of this. Nearly all phrases (segments) of the song resolve to a C Major chord.

Knowing what key a song is in also gives you an idea of what chords you’re most likely to come across. In the above example Cohen uses C Major, A Minor, F Major, G Major, and E Major chords in his song. These are all chords you’d expect to see in C Major but not in, say, A Major.

How key signature is arranged

There are two kind of key signatures: sharp and flat. A sharp key signature will have all sharps, like this:

C# Major’s Key Signature

…and a flat key signature will have all flats, like so:

Key signatures have either all sharps or all flats, or neither; there’s never a mix of the two.

Sharp and flat key signatures follow different, perfectly opposite patterns.

Sharp key signatures

Sharp key signatures follow the pattern: F, C, G, D, A, E, B. A sharp key signature doesn’t need to have all of these, but it must always be arranged in this order, up until the last sharp. Let’s look at an example.

B Major key signature
B Major key signature

In the key signature pictured above, the sharps included are as follows: F, C, G, D and A sharp. As you can see, E# and B# aren’t included.

Nevertheless, this key signature follows the order we mentioned earlier up until the last sharp included (A# in this instance). You will find the same to be true when you look at any sharp key signature.

This means that, if given the number of sharps in a key signature, you can figure out what notes are played sharp.

If there are three sharps they must be F, C, and G sharp. If there are six, they’re F, C, G, D, A, and E sharp.

Flat key signatures

Flat key signatures are arranged in a very similar manner, but backwards. Thus, the pattern is B, E, A, D, G, C, F. A flat key signature must always start with B♭, and proceed through as many flats as it takes in the aforementioned order.

Gb Major key signature
G♭ Major key signature

As shown above (this is the key signature for G♭ Major), all of the accidentals follow this pattern.

As with sharp signatures, you will always be able to identify which notes are played flat by simply knowing the number of flats in the signature.

If there are three, they must be B, E, and A flat, following the pattern. If there are only two, they must be B and E flat.

How do you figure out what key a song is in using key signature?

You might recognize some key signatures at first glance. If you’ve every studied music theory, you’ll probably recognize C Major’s key signature (no sharps or flats), G Major’s (one sharp), and F Major’s (one flat).

If you come across one you don’t recognize by sight, though, don’t worry. It’s incredibly easy to figure out what key a song is written in using only its key signature.

Figuring out the key of sharp key signatures

With a sharp key signature the rule is easy: Identify the last sharp, and find the note a semitone (or half-step) above that. This is the major key signature.

Look at the key signature below:

E Major key signature

The last accidental, circled in blue, is D#. One half-step above D# would be E. Therefore, this key signature tells us that the song is in is E Major.

There’s one caveat to this, though. The key signature could be either E Major or C# Minor (E Major‘s relative minor). You can’t know for sure until you play through the piece, or at least give it a good look.

Regardless of which of these it is, you’ll still play the same notes sharp. The resulting “feel” of the song is the only thing that will be affected.

Figuring out the key of flat key signatures

When looking at a flat key signature, find the second-to-last flat. Figure out what note this is; that’s they key.

Let’s look at an example:

A key signature, A♭ Major in this example

In this key signature there are four flats: B, E, A, and D flat.

The second-to-last one (circled in blue) is A♭. This is the key signature: A♭ Major.

As with sharp key signatures, we can’t know for sure whether this is truly AMajor or its relative minor, F minor, without identifying the tonic.

Finding the Relative Minor

Those tips are great for finding the major key signature, but what if the song you’re looking at is in a minor key?

This, too, is simple to figure out. Find out what the major key the signature indicates, then drop the note down three half steps.

Here’s an example: you see this key signature and know the song you’re trying to play is minor.

D Major/B Minor key signature
D Major/B minor key signature

Using the rule for sharp signatures we covered above, you take the last sharp (C#) and raise it a half-step. This gives you D, so you know that this is D Major‘s key signature.

Take this note and lower it three half-steps: C#, C, and finally B. In other words, the song in question had a key signature of B minor.

The Circle of Fifths: What is it?

Circle of fifths
The Circle of Fifths (without key signatures)

The Circle of Fifths is a concept that ties key signatures together. It explains why the order of flats (remember BEADGCF?) is the exact opposite of the order of sharps (FCGDAEB), and why they’re arranged in such a way.

It’s exactly what it sounds like: A circle of perfect fifths, which are degrees of seven semitones, or the distance from the first to the fifth note in a major scale.

Circle of fifths
The circle of fifths, displayed on a musical staff

The Circle of Fifths begins on C and circles all the way back to C in a continuous loop. The first seven entries in the circle are sharp key signatures, while the last seven entries are flat key signatures (look at the graphic below if that doesn’t click).

The sharp and flat key signatures overlap at B Major/C♭ Major, G♭ Major/F# Major, and C# Major/D♭ Major. These are enharmonic keys (they include the same notes; they’re just written differently).

C Major is considered a natural key, as it has neither sharps nor flats.

The Circle of Fifths
Just plain Bill , 
Circle of fifths deluxe 4 , 
CC BY-SA 3.0

As you can see, the number of sharps increases by one as the circle of fifths moves clockwise, and the number of flats increases as the circle moves counterclockwise, up to the point when there are seven sharps or flats.

This concept is monumentally important because it helps you quickly identify key signatures, especially sharp signatures.

Every time you add a fifth you add a sharp, until you have seven sharps. G Major has one sharp, so a fifth away from this (D Major) must have two. Another fifth gives you A Major, which we know must have three.

This pattern also applies to each key’s relative minor, which is shown in green in the image.

The Circle of Fourths

We can also look at the Circle of Fifths as a Circle of Fourths: Just read it counterclockwise.

Starting from C, we see intervals of fourths instead of fifths: C, F, B♭, E♭, and so on. As with the Circle of Fifths, this can go on indefinitely, as it ends up back on C and starts over again.

The Circle of Fourths is more helpful for identifying flat key signatures than sharps, since one flat is added for each interval of a fourth.

Think about it: B-flat‘s key signature has two flats. Ascend a fourth, and you have E-flat, with one more flat in the key signature (for a total of three). Add another fourth and you have A-flat, with four total flats in its key signature.

Conclusion

We hope this walkthrough of key signatures and the Circle of Fifths (or fourths, depending on how you look at it) has given you a deeper understanding of how music works.

Learning to play in various keys is the first step to becoming a complete musician, and understanding key signature in instrumental in this.

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