Each letter is a key on the on-screen piano below. Lowercase letters are white keys, capital letters are black keys. Open the piano and press them in order — no sheet music needed.
C major (C E G) · A minor (A C E) · F major (F A C) · G major (G B D)C, A minor, F, G is the progression behind a huge share of pop, rock and folk songs. Learning it means you can accompany yourself singing almost immediately. Practise it as four groups of three notes, holding each group and moving only the fingers that need to move.
Four triads in C major: C (C-E-G), A minor (A-C-E), F (F-A-C) and G (G-B-D). It is the most widely used chord loop in popular music.
Because songs are made of them. Once you can move between four chords smoothly you can accompany hundreds of songs instead of learning each one note by note.