Music theory basics
Four ideas explain most of the music you hear: intervals, scales, chords and tension. Play with each one below and hear it as you read.
1. Intervals: the distance between two notes
Everything in music is built from the distance between notes, measured in semitones (one piano key to the next). Two notes that are 12 semitones apart form an octave: the higher note vibrates exactly twice as fast, and our ears hear them as “the same note”. Intervals whose frequencies form simple ratios, like the perfect fifth at 3:2, sound smooth; complex ratios sound tense.
C → G · ratio ≈ 3:2 · 262 → 392 Hz
Sounds strong, stable. Think “Twinkle Twinkle” (1st → 2nd pair).
2. Scales: a pattern of steps
A scale is a recipe of whole steps (W, 2 semitones) and half steps (H, 1 semitone) that climbs from a note to its octave. The recipe, not the starting note, gives a scale its character. Tap the steps below to change the recipe and hear what happens. Keep the total at 12 semitones so the scale lands on the octave.
If you like learning by poking at a working model like this, ahaboo has narrated interactive explainers on how everyday things work, from the seasons to compound interest.
Total: 12 semitones ✓ reaches the octave · This is the Major scale.
Notes: C D E F G A B
3. Chords: stacks of thirds
Play three notes together, each a third apart (3 or 4 semitones), and you get a triad. A major third on the bottom (4) with a minor third on top (3) makes a major chord; flip them and it’s minor. Keep stacking thirds and you get 7th and 9th chords, each adding more colour and more tension.
C – E – G · gaps 4 + 3
4. Tension and resolution: why V wants to go home
Songs move between stable and unstable chords. The dominant 7th chord (V7) is the most unstable chord in a key, and it points straight back to the tonic. Step through the classic I–IV–V7–I cadence and watch the two notes that do the pulling.
C major: every note agrees with the key. Nothing needs to happen next.
Putting it together
A key is a scale plus a sense of home. Build a triad on each note of the major scale using only notes from that scale and you get the seven chords of the key: I, ii, iii, IV, V, vi and vii°. Songs mostly move between these, and the interval patterns above explain why some moves feel satisfying. See how keys relate on the circle of fifths, hear the seven chords in any key with the chord progression generator, or browse every chord on the piano chords chart.
Ask a music theory question
Answers are written by AI and scoped to music. Check anything important against a trusted textbook.
Questions
What is music theory, in one sentence?
It is the set of names and patterns musicians use to describe why notes and rhythms sound the way they do together: intervals, scales, chords, keys and rhythm.
Do I need music theory to write songs?
No, many great songwriters learned by ear. But a little theory (scales, chord building, the I–IV–V functions) makes it much faster to find chords that fit a melody and to understand songs you like.
Why are there 12 notes in an octave?
Western music divides the octave (a 2:1 frequency ratio) into 12 equal semitones because 12 steps give very close approximations of the simple ratios the ear hears as consonant: the fifth (3:2) is 7 semitones, the fourth (4:3) is 5.
What is the difference between a key and a scale?
A scale is a set of notes in order. A key is a scale treated as a home base: the tonic feels like the resting point, and chords built from the scale take on roles (tonic, subdominant, dominant) relative to it.