The most important fact about concerto form is that the audience waits for the soloist to enter, and when he stops playing they wait for him to begin again. In so far as the concerto may be said to have a form after 1775, that is the basis of it. This is why the concerto has so strong and so close a relationship to the operatic aria...essentially what the classical period did was to dramatize the concerto, and this in the most literally scenic way--the soloist was seen to be different. [p. 196]This is one very important reason why Mozart was so very successful in shaping the form of the classical concerto: he was virtually the only composer in music history who was as comfortable writing opera as he was writing instrumental music. In the Baroque concerto, the soloist (or soloists) are part of the orchestra as they played along throughout. They stepped out to take solos, but the really dramatic moments are when the full orchestra comes in. But the Classical concerto is very different: the big event is the entrance of the soloist, akin to the entrance of a character--a flamboyant character--onto the stage.
One of the techniques that Mozart used to structure concerto form was to have two expositions. You might recall that sonata form is essentially ternary:
The theme or themes are presented in the exposition, which, in the symphony and quartet genres, is often repeated. Then these themes are developed in the next section. This is done largely through modulation and the breaking down of the themes into their motifs. Then the recapitulation brings back the material in its original form with the difference that it is now all presented in the tonic where in the exposition, some themes might have appeared in the dominant.
What Mozart does in his concertos is to give us two expositions: one for the orchestra that does not modulate and a second one for the soloist, that does. Let's have a look at his Concerto, K. 450 to see how this might work. Here is the opening theme:
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Sorry to divide it into two lines. This is a regular eight-measure period with the first four measures ending with a half-cadence, followed by another four measures ending with a full cadence. The contrasts inherent in the period (two measures of basic idea followed by two measures of contrasting idea) are intensified by splitting the phrase between the strings and the winds. Indeed, this is one of the first concertos where Mozart makes very full use of the wind instruments. Subsequent themes have a similar harmonic foundation:
The use of the winds enable Mozart to give interest to the orchestral exposition, even though he is going to save the modulation for the piano's exposition. Here is how the piano enters:
That's a seven-measure phrase with brilliant passage-work and different material from what we have heard so far. The piano continues with material we first heard in the winds, interspersed with more brilliant passage-work. Another difference between the orchestral exposition and the one given to the piano is that the piano is constantly in a dialogue, first with the winds, then the strings, of the orchestra. It presents material that the orchestra echoes or comments on. Usually the piano has more elaborate versions of the material. The development is begun by the piano and shared with the orchestra. The recapitulation is also shared, though Mozart treats it, as he does all the formal elements, with great freedom. The theme, originally begun in the winds, is reintroduced by the strings under a trill in the piano:
Mozart dramatizes the concerto form by allowing great freedom and spontaneity to the piano part, giving it an individualized character as contrasted with the group character of the orchestra sections. We hear this throughout the recapitulation where the piano is constantly given solos that take over from and comment on the orchestral parts. The movement, as was normal, culminates in a cadenza for the piano that at the time would have been improvised, but nowadays is usually practiced beforehand. Here is Robert Levin, fortepiano, with the Academy of Ancient Music conducted by Christopher Hogwood: