secondarydominant

A secondary dominant is a dominant function chord that temporarily tonicizes a chord other than the tonic within a key. It is built as the dominant seventh of that target chord and is labeled V7/X, where X is the scale degree of the chord being tonicized. The use of a secondary dominant does not establish a new key; its effect is local and short-lived, returning to the original key when the target chord resolves.

Common targets are diatonic chords such as ii, iii, IV, V, or vi in major keys, and their equivalents in minor keys. For example, in C major, V7/ii is A7 resolving to Dm (ii); V7/V is D7 resolving to G (V); V7/vi is E7 resolving to Am (vi). These progressions create forward momentum by introducing a temporary dominant that leads to the next harmony.

In practice, a secondary dominant contains the leading tone of the target chord as its third, and its seventh typically resolves downward to a chord tone of the target. The V7/X pattern can appear successively (V7/ii–V7/V–V7/I) to create a chain of tonicizations.

Notationally, authors write V7/X or V7/diatonic-chord, and the concept extends beyond major keys to minor keys with appropriate alterations. Secondary dominants are widely used in classical music, jazz, and pop to add color and drive without long-term modulation.