Three-chord songs: I, IV and V

In the key of G the three main chords are G, C and D. Those three are enough for Amazing Grace, whose words were first printed in 1779.

Numbering the chords of a key

Every major key has seven notes. In G major they are G A B C D E F♯. Musicians number them with Roman numerals and build a chord on each. The chord on the first note is I, the home chord. The chord on the fourth note is IV and the one on the fifth is V. In G that gives G, C and D. These three major chords between them contain every note of the scale, which is why a melody in G can almost always be harmonised with them. V pulls strongly back to I, and IV sits between them, moving away from home without the same tension.

Look at the table and notice which keys use only chords you already know. G (G, C, D), D (D, G, A) and A (A, D, E) need no barre at all. C needs F, which you met in unit 3. E needs B, which on the guitar is a barre chord or a four-finger shape, so beginners often move E songs to another key. The progression I-IV-V-I is so basic that music theorists describe it as defining the key itself, and composers have used it to open and close pieces for centuries.

A song in public domain: Amazing Grace

The words of Amazing Grace were written by John Newton, a Church of England curate in Olney, for his New Year’s Day service in 1773, and first printed in the Olney Hymns in 1779. The tune it is sung to today, known as New Britain, was printed in 1829, and William Walker first joined the tune to Newton’s words in The Southern Harmony in 1835. Both are long out of copyright. The song is in 3/4. A simple arrangement in G uses one chord per bar over 16 bars:

Use the waltz strum from the last unit, or a bass note on beat 1 and two light strums. Count "1 2 3" for every bar. The first word, "A-", is a pickup that comes before bar 1, so start strumming G on the syllable "-ma-" that follows it. Arrangements in hymn books add more chords, such as G7 or Em, but these three carry the whole song.

The twelve-bar blues

The same three chords make the twelve-bar blues, a form behind a huge share of blues, rock and roll and jazz. In its most common version the bars go: I I I I, IV IV I I, V IV I I. In the key of A that is A A A A, D D A A, E D A A, all open chords. Play it with four downstrokes per bar and you have the frame that thousands of songs have been built on. Because the form itself is a tradition rather than a single composition, you are free to use it for your own songs.

Try it

Press Play to hear I-IV-I-V in G, the backbone of the arrangement above. Then hum the melody of Amazing Grace over it.