IV. Diatonic Harmony, Tonicization, and Modulation
The Mediant Harmonizing Mi (Scale Degree 3) in the Bass
John Peterson
Key Takeaways
- The iii chord (III in minor) is a weak predominant that typically moves through a strong predominant on the way to a V chord.
- In major, iii usually harmonizes a descending ti
; similarly, in minor, III usually harmonizes a descending te 
. - iii/III is usually found in root position.
- iii is not used as a substitute for I6 (see the explanation below Example 1).
- iii is not not a very common chord.
Overview: the iii chord
It’s most common for mi (
) in the bass to be harmonized with a I6 chord. Occasionally, however, composers choose to use iii rather than I6 (Example 1). The iii chord is used in one relatively specific situation: after a I chord, harmonizing a descending ti/te 
in an upper voice. It commonly moves to to a strong predominant, though it can move directly to V in a bass arpeggiation do–mi–sol 
harmonized I–iii–V, and is usually in root position.
Example 1. The iii chord in Koji Kondo’s Athletic Theme from Super Mario 3 (4:19–4:24).
It’s important to emphasize that iii isn’t simply a substitute for I6 in Western classical music. For instance, in an earlier chapter on tonic prolongations, we saw that the bass line do–re–mi 
is commonly harmonized with 
. Composers don’t use 
as an alternative. That’s because iii functions like vi, as a weak predominant that most often travels through a strong predominant to get to V. The progression shows the opposite: V getting to iii, which isn’t stylistically normative. Finally, keep in mind that iii does not appear very often in common-practice tonality, so it should be used sparingly.
Writing with iii
Example 2 shows the voice leading for the most common use of iii: as a weak predominant that moves through a strong predominant on its way to V. In the major-mode progressions (Examples 2a and 2b), notice that the leading tone descends to la 
when iii moves to the predominant.
In the minor-mode progressions (Examples 2c and 2d), te 
descends to le 
when III moves to a predominant. Notice that III involves te , not ti 
. That is, III is major, not augmented, which is what would happen if we used ti .
Example 2. Writing with iii going to a strong predominant.
Example 3 shows that iii can also go directly to V. Note that this progression doesn’t work well in minor, since III contains te but V contains ti , and the immediate juxtaposition of these two scale degrees is not stylistic for Western classical music.
Example 3. Writing with iii going to V.
in the bass at beginnings (.