Keys and the Circle of Fifths

The idea

A key is a home note plus the scale built on it. “The key of G major” means: G is home, and the notes are those of the G major scale.

Each major key needs a different number of sharps or flats to keep the W-W-H-W-W-W-H pattern. That set is the key signature. Arrange the keys so that each one is a perfect 5th above the last, and you get the circle of fifths:

Click around the circle and watch what changes.

• Clockwise adds one sharp per step: C (none), G (1), D (2), A (3), E (4), B (5), F♯ (6). The new sharp is always degree 7 of the new key.
• Counter-clockwise adds one flat per step: F (1), B♭ (2), E♭ (3), A♭ (4), D♭ (5). The new flat is always degree 4 of the new key.
• Sharps arrive in the order F C G D A E B. Flats arrive in the reverse order: B E A D G C F.

Neighbors share notes. Keys next to each other on the circle differ by a single note. C major and G major share six of their seven notes. That is why moving between neighboring keys sounds smooth, and jumping across the circle sounds dramatic. Module 8 uses this.

The three main chords are neighbors. Pick any key on the circle. The key to its left is its IV chord and the key to its right is its V chord. In G: C on the left, D on the right. G, C and D are the three main chords of the key of G.

Relative keys share all their notes but have different homes. Every major key has a relative minor built on its 6th degree: C major and A minor, G major and E minor. On the circle, the relative minor is written inside each major key. On guitar, the relative minor root is three frets below the major root.

Parallel keys share the home note but not the notes: C major and C minor. Switching between parallel keys changes the color while keeping the center. Module 8 uses this too.

Relationship	Example	Shares
Relative	C major and A minor	All seven notes, different home
Parallel	C major and C minor	The home note, different notes
Neighbor on the circle	C major and G major	Six notes of seven

On your instrument

Guitar keys. Some keys are popular in rock for a physical reason: their main chords use open strings, and their root is the lowest note you can play.

Key	Why guitarists like it
E minor, E major	The open 6th string is the root. The heaviest possible riff key in standard tuning.
A minor, A major	The open 5th string is the root, with open E available as the 5th below.
D minor, D major	The open 4th string is the root. In Drop D the 6th string becomes a low root.
G major, C major	Open chords for I, IV and V, with relative minors E minor and A minor.

This is why so much guitar music is in keys with sharps. Horn-based music prefers flat keys for the same kind of physical reason.

Changing key without changing shapes. A capo moves everything up: shapes in G with a capo on fret 2 sound in A. Tuning the whole guitar down a half step moves everything down: shapes in E sound in E♭. The theory does not change. Only the pitch does.

Piano. Play C major, then G major, then D major. Each new scale keeps all the notes of the previous one except one, which moves up to a black key. You are walking clockwise around the circle.

Worked example

Finding the key of a song from its chords. Suppose a song uses G, C, D and Em.

1. Find the three major chords on the circle: C, G and D sit in a row.
2. The middle one is the home key: G major. C is its IV and D is its V.
3. Em is the relative minor of G, written inside the G segment. It belongs to the same key.

If the song feels sad and keeps landing on Em, the key is E minor, the relative of G major. The notes are the same. The home is different. Your ear decides: hum the note that feels like the end of the song.

Exercises

1. Write the circle of fifths from memory, with the number of sharps or flats for each key and the relative minor inside. Do it once a day for a week. It takes under two minutes by day three.
2. For G, D, A, E and F major: write the scale, the key signature and the relative minor.
3. Take five songs you play. Work out the key of each using the method from the worked example.
4. Play a G major scale, then an E natural minor scale, using the same fingering position. Make each one sound like home by starting and ending on its own root.

Common mistakes

• Confusing relative and parallel. Relative keys share notes. Parallel keys share the root.
• Deciding the key from the first chord. Many songs start away from home. The chord that feels like the end is the tonic.
• Avoiding flat keys forever. A capo or a singer will put you in B♭ or E♭ sooner or later. The shapes are the same, three or four frets away.

Check yourself