Chord Function Categories: The harmonic function of a chord depends upon the position of its root in the scale of the prevailing key. Each step of this scale relates in a different way to the central tonicizing process in the key. Most chords in a harmonization fall into three categories:

• tonic; the harmonic goal, the object of harmonic closure (e.g., I [i] or its substitute, vi [VI])
• dominant; essential to the tonicizing progression (e.g. V-I, V7-I, viio7-I, etc.). V and vii are used interchangeably as dominant chords. All common dominant chords reside in the V9.
• pre-dominant: any chord (except tonic) containing at least one voice that can move by step to a voice in the dominant chord (e.g. ii-V, IV-V, vi-V).

Additional categories of chord function have been proposed by some authorities. Rather than present these proposed categories here, it seems more appropriate to discuss the harmonic contexts in which they occur. These harmonic contexts are common tone substitution, series-of-fifths progression and harmonic prolongation, including that of non-tonic harmony.

Common Tone Substitution: Third relations always involve common tones. A chord can be replaced by another a third above or below it and still retain a trace of the original chord. This substitution follows or replaces the original chord. The last substitution in the example is the classic deceptive pattern in which vi replaces I.