Anyone who has taken a music history or music appreciation course has been told that Franz Joseph Haydn's music has a lot of humor. Yet many people don't catch the humor, other than the big boom in the Surprise Symphony. So, I will walk you through the second movement of Haydn's last symphony, the "London" Symphony no. 104. This Andante in G major is set as a theme and variations, a very basic form. You start with a memorable tune, and embellish it with ornaments, re-orchestration, and changes in mode or meter, one signficant change with each new Variation. But Haydn can't be as simple as that.
First, the theme: it is a rounded binary form, as expected. The first eight measures (a) are a very normal parallel period, with only two surprising sforzandi on the last beats of measures 2 and 6 to startle at all. The next eight measures (b) are also normal, a delightful little digression that leads to a half cadence in the original key. This leads to the return of the 'a' motive, leading us to the logical conclusion that this is a very normal rounded binary form. But the return is not so normal. Instead of another parallel period, this time Haydn composes a contrasting period that cadences most unusually on the subdominant (IV) chord. (Think about the song "Take me out to the ball game." The cadence on 'they don't win, it's a shame.' is likewise on IV.) A developmental section follows, leading to a deceptive cadence before finally providing a true authentic cadence and a short codetta. Rounded binary forms do not normally radically alter the return of the 'a' theme, as Haydn has done. The return of 'a' feels like a type of variation on the original. Hmm, variation.... There is a nice closing section to stabilize the ending of the theme.
Variation I features a switch to the minor mode (G minor). The first four measures are otherwise identical to the theme, except played by winds instead of strings. The second phrase contains a "Surprise"-like gotcha, with full orchestra leaping entering in subito fortissimo. This second phrase is also heavily embellished, almost to the point of making the period contrasting instead of parallel. Hmm, another mini-variation... The variation of the 'b' section takes us to the relative major key of Bb, which is perfectly acceptable as long as it gets us back home to a V chord in G minor, by 9 pm on a school night. Instead, Haydn plays the bad boy, keeping us out in the key of Bb far too long. Sure, he pretends to take us home, by setting up a big half cadence and a grand pause, followed by the return of the 'a' motive. But we are not home, we are still in Bb! This could be a full-fledged development, complete with false return, as if Haydn was composing a sonata form. And yet it does feel like we are in the return of 'a' in our rounded binary form for Variation I.
To confuse the issue further, the 'a' motive in Bb leads to another half cadence, this time in the tonic key. But, it is too big. Haydn dangles the orchestra on a D dominant seventh chord for six full measures, making this feel like a very significant structural division. And the next section returns to the major mode, with an exact replication of the 'a' motive in its original repeated parallel period form (the repeat is a variation though) , beginning Variation II. So, that Bb motive really was the return of 'a', in the wrong key and leading to a tonally open conclusion to Variation I (the half cadence in G). No closing section, no tonic chord.
Variation II has some additional jokes. Instead of leading to a IV chord in the return of 'a', Haydn brings us to the key of bV (Db major), almost as far removed as one can get from the tonic key. A new flute cadenza leads us to the slightly closer key of F# major (#VII) and then the strings lead us to another half cadence and the start of the real return of 'a'. Thus everything before was a false return. This new 'a' (starting in measure 122) follows the same path as the original theme, with the digression to C major and then back again. There are a few ornamental differences, keeping up the idea of mini variations within the larger Theme and Variations form. The closing section is slightly extended, to provide a balanced ending to the whole piece.
Theme and Variation forms usually have many variations, allowing the composer to show off their skilz. Haydn instead takes an approach that combines hybrid form and nested variation. There is a sense of continuous variation, like a passacaglia, with the different versions of the 'a' motive that keep popping up. This is nested within the larger sectional Theme and Variations. The theme and two variations have aspects of a ternary form: G major for the outer sections divided by a G minor section; the middle section is tonally open; and orchestration contrasts the middle section from the outer sections.
The main jokes: we have a real return that feels like a false return because it is in the wrong key, and a false return that feels like a real return because it is in the right key. The false and true returns create a feeling of development within a sonata form, when the form is really a hybrid of theme-n-variations and ternary. The deceptively simple way the movement starts, compared to the complex structure that underlies it.