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There's a part of Tchaikovsky's Sleeping Beauty that I like. And I was wondering if I'm right as far as the theory goes:

Measures 156-159 is in the relative key of G minor, and ends on this key.

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Measures 172-176 is in the given key of B-flat major, and ends on this key.

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Here's what it sounds like. I'm mainly talking about from time 2:38 (minor) vs 2:55 (major)

I'm just wondering if I'm right that 2:36-2:38 is a modulation to the relative key of G minor and how do you know? That part just made me sad and I also noticed it ended on a Gm.

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While it's true this entire movement is in B♭, this portion is really in the key of E♭ starting at the pick-up to m. 145 (2:24) until the return to B♭ around m. 177 (2:56). Notice all of the A♭s effectively turning the key signature of two flats into one with three (and thus E♭ major).

With that said, the music definitely cadences in G minor in m. 159 (2:38). But to my ears, this is more a tonicization than a modulation. What's the difference? We typically say that a modulation reaches a cadence in a new key, and this one does! But remember that so much of a cadence is the preparation; here, we really just have a V–i in G minor. A truer preparation (and therefore a stronger cadence creating a bona fide modulation) could have used a predominant that led into this dominant. To me, the E♭ chord in m. 156 sounds too much like the current tonic to be understood as a predominant setting up a G-minor cadence. Furthermore, the music immediately returns back to E♭, quickly removing any status of "tonic-ness" that G could have had. Really, this G-minor cadence is just a way to end this phrase with a perfect authentic cadence (PAC) that isn't the current tonic.

And speaking of cadences: notice that there's never a PAC in this current tonic of E♭! We only receive weak IACs in m. 151 (2:30) and m. 167 (2:47). This really keeps the momentum up to push us forward into the return of the opening material in B♭.

  • I like to use terms "cadence" and "cadential harmony" to try distinguishing this type of progression that looks like a cadence in terms of treble and bass notes matching the definition, but not really having the structural function. I'm not sure if "cadential harmony" is an excepted term to convey that meaning. – Michael Curtis Mar 22 at 19:31

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