Originally indicating a generally instrumental section or composition, as in the case of the brief instrumental introduction to Monteverdi's opera Orfeo, the symphony came to be the principal serious orchestral form of the later 18th century and thereafter. This later form of the symphony (= Italian: sinfonia) has its immediate origin in the three-movement Italian overture to opera found in the work of Alessandro Scarlatti in the late 17th and early 18th century. The Italian overture opens with a fast movement, followed by a slow movement and a final fast dance-movement in triple metre. The function of the symphony as an overture continued into the second half of the 18th century, to be replaced more generally by its new function as an isolated orchestral form. The classical symphony of Haydn and Mozart is generally in four movements, opening with a sonata-form allegro, followed by a slow movement, a minuet and trio and a rondo finale. With Beethoven the symphony grew in size and ambition, an example followed later by Brahms, Bruckner and others. In the 19th century and into the 20th century the symphony, now much expanded, remained the most respected and demanding form that a composer might tackle. A symphony may loosely be defined as an orchestral composition generally in several movements.