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For the piano/composers: I compose music for the piano in my spare time, and one particular chord that has caught my attention (as one that I like a lot) is the D-F-G chord in C Major.

I have seen Mozart use this chord many times. In particular, his piano sonata in C (No.16 K.545). In both the first and second movements, he uses this chord at the beginning. In the first movement, he plays the I chord, then quickly switches to D-F-G, then back again. I don't know what that chord is. If I had to guess, I would say that it is a V7 chord second inversion without the 3rd? (D-F-G-B, excluding the B). In the second movement, you can see the same thing except in the key of G Major.

Link to the Music Sheet: Sonata in C Sheet Music

What interests me most about that chord in particular, is that it doesn't seem like a full chord change. It seems as though Mozart uses it in the middle of the I chord. It seems like some kind of composing "trick" to me. I have heard in the past about how it is possible to extend chord progressions by switching back and fourth between two chords. For example, ending with V, I, V, I [...] rather than just V, I. Perhaps Mozart was using a technique of some sort to lengthen the I chord and keep it interesting?

And as a last question about the "D-F-G" chord: Is there a particular name for a chord like that? I know that a Csus4 is C-F-G, and Csus2 is C-D-G. I had some thoughts that maybe D-F-G might really be some kind of ii chord variant.

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I agree that it's a V7 second inversion. The mediant of the V isn't required. – Matthew Read Jul 23 '11 at 1:09
Well, also, the mediant of the V is in the right hand in that measure. – user954 Jul 23 '11 at 2:53
Sometimes a chord is a chord and sometimes it's counterpoint. A C chord moving up and back to D F G is more like a double upper neighbor than a chord change. – user955 Jul 23 '11 at 3:14
"C7 second inversion". Dies with confusion – Cody Guldner Apr 18 '13 at 2:41
up vote 7 down vote accepted

There are a few observations that can be made here. First, as others have noted, the sonority is the dominant. Always make sure to take the entire context (including both staves as well as the surrounding sonorities) into account when determining what to call a sonority. In the first movement, as @moberhau mentioned, you do have all members of the dominant seventh chord. The inversion simply facilitates pianistic voice-leading. In the second movement, first measure, again there is a momentary shift to a dominant sonority in the lower staff only. What about entire context? I'm getting to that.

You made an observation that alternating chords can serve just to prolong one harmony. Yes, that is what seems to be happening in both the places you mentioned. The dominant is not truly structural, but rather it prolongs the tonic. There is no larger-scale advancement of the form of the movement that occurs due to the dominant sonority there.

Finally, trust your ears. Even if a particular note is missing or appears not to fit, what does the passage sound like? In both cases you mention, I believe that the sonority sounds like a dominant "in passing," just to prolong the tonic.

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It's called (since you ask ) G dominant 7 with no 3rd. In guitarspeak G7/D(no3)

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