Tertian

In music theory, tertian (Latin: tertianus, "of or concerning thirds") describes any piece, chord, counterpoint etc. constructed from the intervals of (major and minor) thirds. An interval such as that between the notes A and C encompasses 3 semitone intervals (A-B♭-B♮-C) and is termed a minor third while one such as that between C and E encompasses 4 semitones (C-D♭-D♮-E♭-E♮) and is called a major third. Tertian harmony (also called tertiary harmony) principally uses chords based on thirds; the term is typically used to contrast with quartal and quintal harmony which uses chords based on fourths or fifths.

A common triad chord can be regarded as consisting of a "stack" of two thirds. There are four permutations: A major third stacked on a major third creates an augmented triad. A minor third on top of a major third manifests a major triad. A major third on top of a minor third produces a minor triad. Finally, a minor third stacked on a minor third constitutes a diminished triad.

A musical scale may also be analysed as a succession of thirds.

The meantone temperament, a system of tuning that emphasises pure thirds, may be called "tertian".

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