You know scales. Now the focus is keys. A key is a scale in action - it tells you which notes and chords belong together in a piece of music, and which note feels like home. When someone says "this song is in C major," they mean the melody, chords, and bass all draw from the C major scale's pantry, with C as the centre of gravity.
Scale + tonal centre = key
A scale is the pantry - a collection of available ingredients. A key is the cuisine - using that collection with one note acting as the defining flavour, the tonal centre. Everything in the song gravitates toward that centre.
A pattern of intervals (T-T-S-T-T-T-S for major)
That pattern applied from a specific root, used as the harmonic framework for a song
The home note of the key - where phrases want to resolve to
Key signatures: sharps and flats
Each key has a key signature - the number of sharps or flats needed to build the major scale from that root. C major has none. As you move to other keys, sharps or flats appear.
Major keys and their signatures
You don't need to memorise these - the pattern is logical and the circle of fifths (next lesson) makes it click.
Chords that belong to a key
A key doesn't just define which notes to use - it defines which chords belong. Build a triad on each degree of the scale using only notes from that scale, and you get the diatonic chords.
Diatonic triads in C major
Roman numerals show the pattern: uppercase = major, lowercase = minor, ° = diminished. This pattern (Maj-min-min-Maj-Maj-min-dim) is the same in every major key.
Tap each to hear the diatonic triads of C major:
Major key vs minor key
When a song is "in a minor key," the minor scale's root is the tonal centre. The diatonic chords change quality too.
C major diatonic
I ii iii IV V vi vii°
C Dm Em F G Am Bdim
A minor diatonic
i ii° III iv v VI VII
Am Bdim C Dm Em F G
Same notes, but the chord qualities shift when you reorder them around a new root
How to hear a key
The key is defined by where the music wants to rest. If the last chord of a song feels final on C major, the song is in C major. If it feels settled on A minor, it's in A minor - even though they share the same notes.
Shared C-major/A-minor note pool, different cadence target, different key identity.
Key takeaway
A key = a scale + a tonal centre. It tells you which notes and chords "belong" in a piece of music. Key signatures show how many sharps or flats are needed. Diatonic chords are built from the scale's notes. Major and relative minor keys share notes but have different home bases.
Next: a diagram that maps all keys and how they relate - the circle of fifths.
Choose your key in Starts
Select root note + scale to set the key. All generated parts will use only diatonic notes.