Fifths

Fifths, in music theory, describe the interval between two pitches seven semitones apart. The most common form is the perfect fifth, where the lower note is seven semitones below the upper note. In practice, this interval underpins many melodic and harmonic relationships across styles.

In just intonation the frequency ratio of a perfect fifth is 3:2; in equal temperament it is tempered to 2^(7/12), about 1.4983, corresponding to 700 cents. The perfect fifth is one of the most consonant intervals after the octave and is central to tonal perception and tuning systems.

Functionally, the fifth links root and dominant in diatonic scales and is central to harmony and chord construction. Many chords include a fifth above the root, and power chords emphasize the root and fifth. The circle of fifths arranges keys or pitches by successive fifths and guides modulation and voice leading. In traditional counterpoint, consecutive parallel fifths between voices are typically avoided.

Tuning history shows different approaches to fifths; Pythagorean tuning uses pure fifths, while equal temperament compromises make twelve fifths fit an octave, enabling consistent semitone spacing across keys and enabling equal treatment of all keys.

Fifths also appears outside music. In American usage, a fifth refers to a unit of liquor—the volume of a fifth of a gallon; modern bottles are commonly 750 milliliters. The term’s other senses are much less prominent in everyday discourse.