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.

The I, IV, and V chords are called, respectively, the tonic, subdominant, and dominant chords.

The I IV and V chords are generally arranged (in the context of this node) in cadences, made up of two (or more) chords.

V-I is a perfect cadence, and is generally considered to have a "closed" sound. Many songs end with either this cadence or a plagal cadence.

IV-I is an plagal cadence, and is generally considered to have a "closed" sound, but not as strong as the perfect cadence.

IV-V is an imperfect cadence, and is generally considered to have an "open" or unfinished sound. (Some consider I-V as an imperfect cadence too).

See also: degrees of the scale

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