C major (no sharps or flats)

Sharp Keys:

G Major/E minor (1 sharp)

D Major/B minor (2 sharps)

A Major/F-sharp minor (3 sharps)

E Major/C-sharp minor (4 sharps)

B Major/G-sharp minor (5 sharps)

F-sharp Major/D-sharp minor (6 sharps)

C-sharp Major/A-sharp minor (7 sharps)

Flat Keys:

F Major/D minor (1 flat)

B-flat Major/G minor (2 flats)

E-flat Major/C minor (3 flats)

A-flat Major/F minor (4 flats)

D-flat Major/B-flat minor (5 flats)

G-flat Major/E-flat minor (6 flats)

C-flat Major/A-flat minor (7 flats)

These are all the possible keys you can write in, enharmonic keys are italicised.