Although the term 'classical' is commonly used to describe all orchestral music, in European musicology it has a quite specific meaning, referring to the period between about 1750 and 1820. By the end of the 18th Century, the instrumental line-up of the orchestra had become standardised as follows:
2 (or 4) Horns
The continuo favoured by Baroque orchestras was abandoned and keyboard instruments disappeared. String sections would vary in size but the minimum numbers were reckoned to be 6 first violins, 6 seconds, 4 violas, 4 cellos and 3 double basses, whereas the maximum could be as many as 28 violins in total, with a corresponding increase in the number of other strings. Amongst the woodwinds, the comparative weakness of sound meant there was often duplication of parts. This affected mostly the bassoons (four bassoons to one part was not uncommon), and there were rarely more than two flutes or oboes. Additional 'exotic' instruments (sometimes known as 'Turkish' instruments) were brought into the orchestra for specific pieces, often in opera. These included piccolo, trombones, harp and some percussion (e.g. triangle, tambourine, cymbals).
The classical orchestra became one of the major vehicles for the increased interest in formal clarity and development during this period. The new approach to composition also allowed for the development of some strong orchestral effects. These first appear in the works of Johan Stamitz, and the rest of the Mannheim school of composers, with their 'sky rockets' (an upward leaping arpeggio) and 'sighs' (a falling phrase). The three major figures of the Classical period, Haydn, Mozart and Beethoven, developed orchestral composition to the point where the orchestra became the grandest and most powerful tool of musical expression.
Haydn experimented with several additions to the standard line-up, including trombones and contrabassoon. Mozart tended to work with the standard orchestra, concentrating on refining its musical possibilities. Beethoven, of course, was greatly influenced by the technological innovation of the pianoforte and the dramatic qualities of that instrument were often reflected in the way he handled the orchestra. From his Fifth Symphony onwards, trombones were established as regular orchestral members.
Developments in instrument technology and design coincided with the shift towards Romanticism in Europe, and the next phase of development brought the orchestra close to the configuration we know today.