Audio: secondary triad (0:08)
secondary triad plays primary and secondary triads on a clavinet in the key of C. The secondary triad figure shows the score.
A secondary triad plays a support role to a primary triad in the promotion of tonality.
There are four secondary triads. They are all built on a degree of the scale other than the tonic, subdominant or dominant. In other words, they are built on the second, third, sixth and seventh degrees of a scale. In a major scale the secondary triads are notated ii, iii, vi and viio, in a minor scale they are iio, III, VI and viio.
A secondary triad has multiple functions: it can replace a primary triad, it can act as a predominant chord, and it can have a general function in support of the harmony. These functions can overlap.
A secondary triad can function as a replacement for a primary triad. This is probably its most common use. A secondary triad that shares two notes in common with a primary triad can replace it. In a major key, iii and vi both function as a replacement for the primary triad I, as shown in the first bar of secondary triad. viio functions as a replacement for the dominant triad V, as shown in bar 2. ii can replace the primary IV triad, as shown in bar 3. Secondary triads in a minor key work in the same way: III and VI replace the tonic i; VII the dominant V; and iio the subdominant iv. Somewhat confusingly, the terms, relative chord and parallel chord, are occasionally used to describe the replacement function of a secondary triad, which implies that modulation is taking place. This is not the case, the key remains the same, and the secondary triad functions as an alternative to the primary triad.
A secondary triad can also function as a predominant chord. This is a generic term for any chord that precedes a dominant V chord. This role is evident in bars 3-4 of secondary triad in the I-iib-V-I chord progression where the supertonic triad, iib, functions as a predominant.
Finally, a secondary triad can also have a general function in support of the harmony. This is a somewhat vague role. It can reinforce motion, a typical example being the use of a secondary triad in a chord progression such as i-iv-VII-III, which reinforces motion by a fifth down. Another role it performs is to harmonise a given melody, a function it shares with a primary triad. It can also function as decoration by adding variety to a chord progression consisting largely of primary triads.
The multi-functional secondary triad is an all-purpose chord and its functions are more varied than those of the primary triads. The function of the subdominant IV or iv is noticeably ambiguous, sometimes it functions as a primary triad, other times as a secondary triad.