V. Diatonic Harmony, Tonicization, and Modulation

Mi (scale degree 3) in the bass at beginnings

John Peterson

Key Takeaways

The iii chord is a weak pre-dominant that typically moves through a strong pre-dominant on the way to a V chord.  It usually harmonizes a descending leading-tone (or subtonic in minor), and it’s usually found in root position. It’s not used as a substitute for I6 (see the explanation below Example 1), and it’s not a very common chord.

Overview: the iii chord

It’s most common for Mi (\hat{3}) 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 relatively specific situations:

  1. It almost always comes after a I chord
  2. It always harmonizes a descending leading tone (or subtonic in minor) in an upper voice
  3. It most commonly moves to a strong pre-dominant, though it can move directly to V in a bass arpeggiation Do–Mi–Sol (\hat{1}\hat{3}\hat{5}) harmonized I–iii–V
  4. The iii chord usually shows up in root position

Example 1. The iii chord in Koji Kondo’s Athletic Theme from Super Mario 3.

It’s important to emphasize that iii can’t substitute for one six in western classical music. For instance, in an earlier chapter we saw that the bassline Do-Re-Mi (\hat{1}\hat{2}\hat{3}) is commonly harmonized with I–V\begin{smallmatrix}4\\3\end{smallmatrix}–I6. Composers don’t use I–V\begin{smallmatrix}4\\3\end{smallmatrix}–iii as an alternative. That’s because iii functions like vi, as a weak pre-dominant that most often travels through a strong pre-dominant to get to V. The progression I–V\begin{smallmatrix}4\\3\end{smallmatrix}–iii shows the opposite: V getting to iii, which isn’t stylistically normative. Finally, we should emphasize once more that iii is not a very common harmony, and we recommend using it sparingly in Western classical music.

Writing with iii

Example 2 shows the voice leading for the most common use of iii: as a weak pre-dominant that moves through a strong pre-dominant 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 pre-dominant. In the minor mode progressions (Examples 2c and 2d) notice that III involves Te (\downarrow\hat{7}), not Do (\hat{1}). That is, III is major, not augmented, which is what would happen if we used Ti (\uparrow\hat{7}). Te (\downarrow\hat{7}) descends to Le (\downarrow\hat{6}) when III moves to a pre-dominant.

Example 2. Writing with iii going to a strong pre-dominant.

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 (\downarrow\hat{7}) but V contains Ti (\uparrow\hat{7}), and the immediate juxtaposition of these two scale degrees is unstylistic.

Example 3. Writing with iii going to V.

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Open Music Theory by John Peterson is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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