Music Theory Basics
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What is a key?
A key is a set of notes that sound good together, centred around a home note called the tonic. The key of C major uses the notes C, D, E, F, G, A, B. The key of A minor uses the same notes but starts on A. Keys are what give music its sense of "home".
The circle of fifths
The circle of fifths is a map of all 12 keys arranged by how many sharps or flats they have. Moving clockwise adds a sharp; counterclockwise adds a flat. Keys that sit next to each other on the circle sound similar and modulate smoothly to one another.
- 0 sharps/flats: C major / A minor
- 1 sharp: G major / E minor
- 2 sharps: D major / B minor
- 1 flat: F major / D minor
- 2 flats: Bb major / G minor
Chord progressions and Roman numerals
Chords in a key are labelled with Roman numerals: I, ii, iii, IV, V, vi, vii° (uppercase = major, lowercase = minor, ° = diminished). The most common progression in pop music is I–V–vi–IV. In C major that's C–G–Am–F.
- I–IV–V — blues, rock, classic
- I–V–vi–IV — pop, indie, film
- vi–IV–I–V — sensitive, emotional
- I–vi–IV–V — doo-wop, 50s
- ii–V–I — jazz standard
What is a capo?
A capo is a clamp you attach to the guitar neck to raise the pitch of all strings. It lets you play open chord shapes in different keys without learning new grips. Capo 2 with G, C, D shapes actually plays in the key of A.
Major vs minor feel
Major keys generally sound bright, happy, or triumphant. Minor keys sound dark, sad, or dramatic. Natural minor is the most common minor sound. Harmonic minor adds tension with its raised 7th. Dorian mode is a minor mode with a brighter 6th — common in funk and soul.
Building a chord progression
To write a song:
- Pick a key (start with C major or A minor if you're learning)
- Choose a progression (try I–V–vi–IV)
- Play the chords and hum or sing a melody using notes from the matching scale
- Experiment — swap chords, change the rhythm, add suspensions for colour
Power chords
A power chord is just a root and a 5th (no 3rd). Without the 3rd, it's neither major nor minor — it's heavy, aggressive, and works with distortion. The foundation of rock and metal rhythm guitar. Often written as C5, G5, etc.
Use the chord & scale player to experiment with everything above. Tap a root note, then tap a chord to hear it. Try the scales wheel to find melodies that fit.