A new form developed at this time, called sonata form, is closely associated with the symphony — even though it turns up in much other music in other genres. The opening movement of every symphony is in sonata form, and this movement counts as the intellectual and emotional core of the whole work. Many Classical works have two or even three movements in this same form.

The reason for this wide use, perhaps, was that more than any other form, sonata form exploited what was the overriding interest of Classical composers. Their interest was in contrasts of every kind — especially contrast of musical themes and contrast of key, or tonality. Composers found that sonata form enabled a special flexibility of expression. They could use it for forceful, brilliant, pathetic, even tragic opening movements, gentle or dreamy slow movements, and lively, often humorous closing movements.

Viewed on the highest level, sonata form is simple enough — a very large-scale example of A B A′ form, usually with repetitions: |: A :||: B A′ :| or |: A :|| B A′. What is less simple, and what makes sonata form different from other A B A forms, is the nature and the function of the musical material in each letter section. This is implied by the special names given to them: A is called the exposition, B the development, and A′ the recapitulation. What do these terms signify?