The classical concert, seen in its aspect as an instrumental solo in struggle against the mass of an orchestra, would have its origin in the solo concert of the Baroque period. Its three movements (moderately fast: slow: fast) correspond to those of the symphony, but without the minuet. The first movement, however, is in a modified sonata form, which begins with a "double exposure": one for the orchestra alone, featuring the main musical material all in the key. Then come the soloist and a second exhibition, now with the second theme in a related tone.
In short, double exposure is the term that is used when a soloist and orchestra takes turns playing the themes of an exhibition.
