In music and musical analysis, a subsidiary chord is an elaboration of a principal harmonic chord in a chord progression.
If the principal chord (X) is partially replaced by the subsidiary (Y), there are three possible positions - beginning, middle, and end - for the subsidiary:[1]
X–Ya
Y–X
X–Y–X
For example, a subsidiary chord in a modulation.
A subsidiary chord may be a chord with related function and/or sharing pitches, for example in E major, Cm (C-E-G) as a subsidiary for E (E-G-B), which share two of three pitches and are related as tonic parallel (vi) and tonic (I).