information:extraction, linguistic:voice_change
Organization

Among the instrument groups and within each group of instruments, there is a generally accepted hierarchy. Every instrumental group (or section) has a principal who is generally responsible for leading the group and playing orchestral solos. The violins are divided into two groups, first violin and second violin, with the second violins playing in lower registers than the first violins, playing an accompaniment part, or harmonizing the melody played by the first violins. The principal first violin is called the concertmaster (or orchestra "leader" in the U.K.) and is not only considered the leader of the string section, but the second-in-command of the entire orchestra, behind only the conductor. The concertmaster leads the pre-concert tuning and handles musical aspects of orchestra management, such as determining the bowings for the violins or the entire string section. The concertmaster usually sits to the conductor's left, closest to the audience. There is also a principal second violin, a principal viola, a principal cello, and a principal bass.

The principal trombone is considered the leader of the low brass section, while the principal trumpet is generally considered the leader of the entire brass section. While the oboe often provides the tuning note for the orchestra (due to a 300-year-old convention), there is generally no designated principal of the woodwind section (though in woodwind ensembles, the flute is often the presumptive leader). Instead, each principal confers with the others as equals in the case of musical differences of opinion. Most sections also have an assistant principal (or co-principal or associate principal), or in the case of the first violins, an assistant concertmaster, who often plays a tutti part in addition to replacing the principal in their absence.

A section string player plays in unison with the rest of the section, except in the case of divided (divisi) parts, where upper and lower parts in the music are often assigned to "outside" (nearer the audience) and "inside" seated players. Where a solo part is called for in a string section, the section leader invariably plays that part. The section leader (or principal) of a string section is also responsible for determining the bowings, often based on the bowings set out by the concertmaster. In some cases, the principal of a string section may use a slightly different bowing than the concertmaster, to accommodate the requirements of playing their instrument (e.g., the double-bass section). Principals of a string section will also lead entrances for their section, typically by lifting the bow before the entrance, to ensure the section plays together. Tutti wind and brass players generally play a unique but non-solo part. Section percussionists play parts assigned to them by the principal percussionist.

In modern times, the musicians are usually directed by a conductor, although early orchestras did not have one, giving this role to the concertmaster or the harpsichordist playing the continuo. Some modern orchestras also do without conductors, particularly smaller orchestras and those specializing in historically accurate (so-called "period") performances of baroque and earlier music.

The most frequently performed repertoire for a symphony orchestra is Western classical music or opera. However, orchestras are used sometimes in popular music (e.g., to accompany a rock or pop band in a concert), extensively in film music, and increasingly often in video game music. Orchestras are also used in the symphonic metal genre. The term "orchestra" can also be applied to a jazz ensemble, for example in the performance of big-band music.