The III and VI degrees have weak harmonic functions. But they are useful when harmonizing.

The VI can have both tonic and subdominant functions since it shares two notes with both chords. Something similar happens with the III degree that shares notes with both tonic and dominant chords.

In the next table, we summarize the discussed chord functions:

dominant			VII
	V
					III	weak
tonic	I
					VI
subdominant	IV
			II

 

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