A symphony is a musical composition, often extened and usually for orchestra. "Symphony" does not imply a specific form. Although many symphonies are tonal works in four movements with the first in sonata form, and this is often described by music theorists as the structure of a "classical" symphony, even some symphonies by the acknowledged classical masters of the form Joseph Haydn, Wolfgang Amadeus Mozart and Ludwig van Beethoven do not conform to this model.