I, IV, and V (and their variants, i, iv, and v) are the chords that form the backbone of music in Western culture. The roman numerals represent the relationship of the chord to the tonic. Capital letters represent major chords, lower-case represent minor.

For example, in the key of C, the I, IV, and V chords would be C, F, and G, respectively. In A minor, the i, iv, and V would be Am, Dm, and E respectively.

In harmonically simple styles of music (folk, rock, children's songs, etc.), the majority of the songs are written with just these chords.