A symphony is a long, complex musical composition for orchestra or similarly capable ensemble that does not feature a soloist and that follows a certain rough structural outline. Having no soloist distinguishes the symphony from the concerto, and having a certain kind of structure distinguishes the symphony from the symphonic poem or suite.
Most commonly, a symphony is written as four movements, with the first movement being in sonata form. Sonata form consists of an optional introduction, followed by an exposition of thematic material, followed by a developmental section during which the previously introduced themes are expanded upon, followed by a recapitulation of the thematic material, followed by an optional coda.
It could be argued that the symphony is the quintessential form of classical music. At minimum, it is one of the most important. Composers have used various other forms to produce interesting and complex music, but the symphony has proven to be a particularly good one. Why this is the case is a subject for scholars, but three centuries of music has left us with no doubt.
There might yet be undiscovered forms of music that offer composers the same kind of expressive power as that of the symphony, but, lacking those forms, Bruckner was still able to create his Eighth, Dvorak his Ninth, Mozart his 41st, and Beethoven his Fifth. Given that these, some of the greatest works of music known to the world, were written as symphonies, we should be careful not to view established forms as being generally restrictive to the creative mind. Indeed, providing a composer with an effective high-level structure to follow reduces the problem of composing to a more manageable size; approaching the composition of a symphony, the composer has a head start on figuring out either how to tell a certain story or how to effect a certain emotion. The alternative approach of devising an entirely new form for each composition does not seem promising if the composer's ambitions lie above simply creating new musical forms.
So, let us celebrate the symphony. And, let us also tip our hats to Joseph Haydn, the first master of the form.
