The musical form in the theme of the Sonata K 331 by Mozart

A small musical analysis, useful to understand the music and play the piano well

In the form "Theme and Variations", the theme is a short piece done, we can call "song form" (or, in German, Liedform), the song form can be a binary form or ternary form.

The song form simplest is usually a binary form, in sixteen bars, which can be divided into two periods of eight bars each and every one of them can be divided into two phrases of four bars each. Therefore: (4 +4) + (4 +4) = 16.

In the analysis of the melody we can see striking similarities between the phrases: the second phrase can be so similar to the first, if we call 'a1' the first, the second can be called 'a2', and with this series 'a1' + 'a2' (4 +4) is complete for the period, which is usually defined by the repeat sign.

In the second period, the third phrase is often a little contrasting (we will call therefore the 'b'), while the fourth usually shows the state of the first or, more often, of the second, so 'a2'.

The binary form is thus characterized often (not always!) by the sequence 'a1-a2-b-a2'.

Another thing is the ternary song form, in which we recognize three periods (eg 8 +8 +8 = 24 bars).
But that is not the case in our Sonata K 331.

Compare with literary forms

We understood from this brief description that the music, especially in the "classic" style, follows its own rules and formal structures, neither more nor less than what happens in literary forms.

The theme of the Sonata K 331

Now back to our K331 theme: we notice that bars 1-4 are the phrase 'a1', bars 5-8 are the phrase 'a2' (almost equal to the first, just change the cadence), and both are the first period. Then the bars 9-12 constitute a situation a bit different ('b', but we could also say 'a3', noting the melodic structure very similar) and then bars 13-16 represent the recapitulation of 'a2'. Happening here is the surprise: we find two additional bars (17-18), added at the bottom of the form.

Freedom and the rule in the art of music

The question now is: why Mozart wrote a a binary song-form and therefore adds two bars and makes it 18 instead of 16? The answer is all too simple: why is Mozart! Or rather, because the rules of composition are never rigid constraints facing the objective of artistic creativity that is always played on the dialectic between rules and freedom.