Tritone Substitution - Substitute Dominant

Substitute Dominant

A substitute dominant is a chord substitution for a chord with a dominant function. Barbara Barry explains:

In the hierarchical organization of key, other degrees may be regarded as having dominant function, for example, the mediant, which can act as a 'second order' dominant, the supertonic, as supportive as a secondary dominant, or the leading note triad. Their ability to define key, though, is less strong than that of the dominant, as is clear when the dominant is present.

However, David Epstein disagrees:

... suggest that the mediant served as a 'substitute' dominant. Perhaps so, as a locale for stating subsequent themes. As a substitute tonal function, however, the mediant-as-substitute-dominant theory seems improbable, given the tendencies of tonal behavior. For in major keys the mediant major is twice indirectly removed from the tonic, only three pitches out of seven being common to the two keys.