The most important element that defines a piano concerto is the instrumentation; a piano concerto is written for a piano soloist with an orchestra. Classical and traditionally structured piano concertos generally have three movements, although since the time of Beethoven it has become more common to include an additional, fourth movement. More recently, composers have written concertos in other forms, including single-movement works, but all concertos meet the basic requirement of a having a piano soloist and an orchestra.
The first movement of a piano concerto is traditionally cast in sonata form with an added cadenza. Classical sonata form comprises three parts: exposition, development and recapitulation; in a concerto, however, right before the recapitulation a cadenza is added; a cadenza is an opportunity for the pianist to show off her technique and improvisational skills, during which the orchestra generally stops playing to allow the pianist to play freely. After the cadenza, the main theme returns (the recapitulation) and the orchestra rejoins the soloist to play the end of the movement.
The second movement of a concerto is traditionally a slow movement, often infused with rubato. Many second movements are lyrical, pastoral or songlike in nature, and cast in a simple da capo aria or rounded binary form. However, the form of the slow movement is more flexible than the first, and may be anything that the composer wishes as long as the piano remains the primary focus of attention.
The third movement of a concerto often uses some type of rondo form; a rondo intersperses repeated material with different, contrasting sections of music. A rondo might, for example, take the form "ABACA" in which the A sections all contain the same material or melody and the B and C sections are entirely new music. In concertos, typically, such rondos are a structured as a call and response, in which the piano will play an idea and the orchestra will respond.
While piano concertos are often cast in the standard three-movement form, several composers have diverged from this model. The concertos of Mozart, for example, usually followed this convention, while Beethoven added a fourth movement, or even new sections within the movements, to fulfill a dramatic rather than a simply formal interpretation of concerto structure. Modern composers who write piano concertos use a great variety of formal designs from single-movement works to more traditionally organized pieces and everything in between.
- "Mozart's Piano Concertos: Text, Context, Interpretation "; Neal Zaslaw; 1997
- "The Concerto"; Ralph Hill; 1978
- "Classical Form"; William E. Caplin; 2000
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