# Tertian

A progression of tertian chords in a minor (i-v-i-iv${}^6_4$-i-v-I) ending on a Picardy third
"Tertial" redirects here. For the feathers in a bird's wing see tertials. For the disease formerly known as tertian fever see Malaria.

In music theory, tertian (Latin: tertianus, "of or concerning thirds") describes any piece, chord, counterpoint etc. constructed from the interval of a (major or minor) third.[1] 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 (See Major and minor).

Quartal chord on A equals thirteenth chord on B, distinguished by the arrangement of chord factors  .[2]

A common triad chord can be regarded as consisting of a "stack" of two such intervals. A musical scale may also be analysed as a succession of thirds (See Ladder of thirds). The meantone temperament, a system of tuning that emphasises pure thirds, may be called "tertian".

Chords built from sixths may also be referred to as tertian because an ascending sixth is equal to a descending third and vice versa: any sixth can be taken as the inversion of a third. For instance the interval C-A is a major sixth that, when inverted, gives the interval A-C, which is a minor third.

Tertian concepts have been used innovatively in chord progressions, as for example in the "thirds cycle" used in John Coltrane' Coltrane changes.