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 [latex](\hat7)[/latex]; similarly, in minor, III usually harmonizes a descending te [latex](\downarrow\hat7)[/latex].
- iii/III is usually found in root position.
- iii is not used as a substitute for I6 (see the explanation below ).
- iii is not not a very common chord.
Overview: the iii chord
It’s most common for mi ([latex]\hat{3}[/latex]) in the bass to be harmonized with a I6 chord. Occasionally, however, composers choose to use iii rather than I6 (strong predominant, though it can move directly to V in a bass arpeggiation do–mi–sol [latex](\hat1-\hat3-\hat5)[/latex] harmonized I–iii–V, and is usually in root position.
). The iii chord is used in one relatively specific situation: after a I chord, harmonizing a descending ti/te [latex](\hat7/\downarrow\hat7)[/latex] in an upper voice. It commonly moves to to a
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 [latex](\hat1-\hat2-\hat3)[/latex] is commonly harmonized with [latex]\mathrm{I-V^4_3-I^6}[/latex]. Composers don’t use [latex]\mathrm{I-V^4_3-iii}[/latex] 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 [latex]\mathrm{I-V^4_3-iii}[/latex] 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
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 ( and ), notice that the leading tone descends to la [latex](\hat{6})[/latex] when iii moves to the predominant.
In the minor-mode progressions (
and ), te [latex](\downarrow\hat{7})[/latex] descends to le [latex](\downarrow\hat{6})[/latex] when III moves to a predominant. Notice that III involves te [latex](\downarrow\hat{7})[/latex], not ti [latex](\uparrow\hat{7})[/latex]. That is, III is major, not augmented, which is what would happen if we used ti [latex](\uparrow\hat{7})[/latex].
shows that iii can also go directly to V. Note that this progression doesn’t work well in minor, since III contains te [latex](\downarrow\hat{7})[/latex] but V contains ti [latex](\uparrow\hat{7})[/latex], and the immediate juxtaposition of these two scale degrees is not stylistic for Western classical music.