Chords combine into progressions, and progressions are the structural backbone of virtually every song. The most fundamental relationship is the I-IV-V progression, where I is the tonic, IV is the subdominant four steps up, and V is the dominant five steps up. In the key of G major, this is the famous G-C-D progression — one of the first formulas beginners learn because it appears in countless pop, rock, country, and folk songs. In E major, the same I-IV-V-I is E-A-B-E, and the I-IV-V7-I version (E-A-B7-E) leans into blues territory. The 12-bar blues is built on this same I-IV-V structure: in E it is E-E-E-E, A-A-E-E, B7-A-E-B7, and in A it is A-A-A-A, D-D-A-A, E7-D-A-E7. Each chord symbol represents one bar of music, and the B7 turnaround in the 12th measure pulls the ear back to the top of the form. Because A, D, and E7 are all friendly open chords, the 12-bar blues in A is a wonderful starting point for beginners.
Beyond I-IV-V, a few other progressions dominate popular music. The vi-IV-I-V progression (Am-F-C-G in the key of C major), sometimes called the "pop-punk" or "sensitive" progression, mixes minor and major colors for an emotionally complex feel — that melancholy comes from opening on the minor vi chord. Songs like "Let It Be," "No Woman No Cry," and "With or Without You" use variants of the I-V-vi-IV progression (G-D-Em-C in G), prized for its satisfying blend of major brightness and minor depth. In jazz, the king of progressions is ii-V-I, which in C major is Dm-G-C; the ii chord provides smooth voice leading into the V, which then resolves cleanly to the I.
Every chord you play in a song usually belongs to the key's chord family — the set of chords built from each scale degree of the major scale. The pattern of qualities is fixed across all major keys: I, IV, and V are major; ii, iii, and vi are minor; vii° is diminished. So in G major the family is G, Am, Bm, C, D, Em, F♯dim; in C major it is C, Dm, Em, F, G, Am, Bdim; and in D major it is D, Em, F♯m, G, A, Bm, C♯dim. Knowing the chord family of a key lets you predict likely chord changes, transpose progressions to other keys quickly, and even improvise harmonically. Once you understand the family pattern, you can construct the family of any major key by stacking thirds and applying the same major/minor/diminished pattern.