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Consider the following cadence from the 3rd movement of Mozart's piano sonata KV 332:

Mozart KV 332, mvmt. 3, mm. 225–232

What is the function of the diminished 7th chord B D F Ab?

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I would offer a slightly different analysis than Aaron's.

This is a B fully diminished seventh chord. In tonal music, we typically encounter fully diminished seventh chords as viio7 chords, so let's try and understand how this particular harmony can function as a viio7.

Thinking through this, B is the leading tone (that is, vii) of C. As such, this chord is viio7 of C; we say that it is "viio7 of V" (labelled viio7/V) because V in F is C.

The extra tricky bit here is that this chord doesn't immediately resolve to V as we might expect, but to this six-four chord. But in music of this style, we understand that this "I64" chord is really a V chord with two non-chord tones up on top, and thus this chord actually does resolve to V. For more on this issue, see Analyzing an unfamiliar Roman numeral analysis: Cm/G-G notated as V6/4-5/3?.

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This is a common-tone diminished seventh chord. Although spelled like a B diminished chord — to illuminate the chromatic, ascending bass line — but its function is to reinforce F, both in the following 6-4 chord and, eventually, the resolution to the I chord.

In light of Richard's viio7/V interpretation — valuable to understand — it's worth mentioning why I choose the c.t. diminished chord interpretation instead. Mozart establishes the pitch F in the initial measure of the excerpt, and every chord in the passage until the arrival on V7 contains that pitch. My feeling is that the primary function of the diminished chord is to prolong the F rather than to reinforce the arrival of C.

For more on c.t. diminished chords, see A chord progression from Leavitt: how to analyze the diminished chord

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    I almost always agree with you, but I'd just call it vii°7 of V, given the spelling and the bass line. I don't feel it particularly reinforces the tonic. Feb 8, 2022 at 13:16
  • @Bennyboy1973 viio7/V is a good interpretation. My feeling is that the chord's primary purpose is to prolong the (pitch) F from the previous several bars into the next.
    – Aaron
    Feb 8, 2022 at 13:20
  • Yeah, I'd say the fact that the chord is immediately preceded by a IV chord with D in the melody wipes out any idea that the chord prolongs the tonic note when it's already deceptively hard to hear in the preceding chord.
    – Dekkadeci
    Feb 8, 2022 at 13:37

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