Chromatic mediant

"[1] A chromatic mediant relationship defined conservatively is a relationship between two sections and/or chords whose roots are related by a major third or minor third, and contain one common tone (thereby sharing the same quality, i.e. major or minor).

[2] Other less conservative theorists, such as Benward and Saker, include these additional chords of opposite quality and no shared tones in their default definition of chromatic mediants.

Chromatic mediants are usually in root position, may appear in either major or minor keys, usually provide color and interest while prolonging the tonic harmony, proceed from and to the tonic or less often the dominant, sometimes are preceded or followed by their own secondary dominants, or sometimes create a complete modulation.

[1] Chromatic mediant chords were rarely used during the baroque and classical periods, though the chromatic mediant relationship was occasionally found between sections, but the chords and relationships became much more common during the romantic period and became even more prominent in post-romantic and impressionistic music.

[1] One author describes their use within phrases as, "surprising," even more so than the deceptive cadence, in part due to their rarity and in part due to their chromaticism (they come from 'outside' the key),[5] while another says they are so rare that one should first eliminate the possibility that one is looking at a diatonic movement (presumably, borrowing), then make sure that it is not a secondary chord, and then, "finally," one may consider, "the likeliness of an actual chromatic mediant relationship.