Parallel keys
Compare a major key with the minor key on the same tonic and name what changes.
One tonic, two modes
C major and C minor start on the same note, C. Keys that share a tonic are called parallel keys. Unlike relative keys, they have different notes and different key signatures.
C major: no signs
C minor: three flats, B♭, E♭, A♭
Compare the degrees: in the natural minor, degrees III, VI and VII are a semitone lower than in the parallel major. In C minor they are E♭, A♭ and B♭ instead of E, A and B. The other degrees, I, II, IV and V, are shared.
Don't confuse parallel and relative keys: relative keys share a key signature, parallel keys share a tonic.
Practice: The parallel minor
Practice: What changes
Hearing the change of mode
The difference is clearest on degree III. Play a triad on C: with E it is major and bright, with E♭ it is minor and dark. One note changes the whole character.
The tonic triad of C major, then of C minor