The Classical orchestra was relatively unchanged from that of the Baroque apart from the trend towards larger ensembles that carried on right through to the modern period; in the Classical orchestra this was largely seen in an increase in the string section.
Out of the instrumental consorts of the Renaissance and Baroque, grew the various string, wind, and brass ensembles that became popular during the Classical period. The most established of these ensembles was the string quartet, consisting of two violins, viola, and violoncello, for which many composers wrote music. As Baroque ensembles grew into larger Classical orchestras the violin underwent a series of changes in its construction to allow it to flourish in the concert hall setting, as discussed in the Baroque section. The neck of the violin was lengthened slightly and set back at a slight angle, increasing the height of the bridge and the pressure put on the body of the violin. With this increased pressure came a louder sound, enabling the violin to be heard across large halls within a group and as a solo virtuoso instrument. Many surviving instruments of the time were altered in this way, and it is now rare to find a violin in its unaltered pre-classical state.
One instrument that became a regular member of the orchestra for the first time in the Classical period was the clarinet, which had been invented in the early eighteenth century, but was popularised in the works of Mozart, Haydn and other composers of the period. A type of clarinet with extended range, the basset horn, was particularly favoured in solo works by Mozart – largely due to his working relationship with one of its most famous players, Anton Stadler.
Larger and smaller ensembles of matched wind instruments were also common – trios, quintets, sextets and octets – with a broad range of music written for these ensembles. During the Classical period valves and pistons found on brass instruments such as the trumpet or horn had not yet been invented. Instead the player had to add sections of tubing, called ‘crooks’ to the instrument to make the overall tubing longer, thus lowering its pitch. At the same time a technique developed on the horn, in which the player puts his/her hand in the bell, altering the sound and allowing him/her to play more notes. You may have noticed horn players with their hand in the bell before!
The piano gained popularity from the last quarter of the eighteenth century, as a solo instrument, for song accompaniment, in chamber ensembles and with orchestra (although usually in this capacity as a virtuoso solo instrument for concertos). Despite this, the harpsichord was still often used in opera performances because of its particular bright and slightly percussive qualities that suited it to leading the opera production.