An orchestra is an instrumental ensemble, usually fairly large with string, brass, woodwind sections, and possibly a percussion section as well. The term orchestra derives from the name for the area in front of an ancient Greek stage reserved for the Greek chorus. The orchestra grew by accretion throughout the eighteenth and nineteenth centuries, but changed very little in composition during the course of the twentieth century.
A smaller orchestra (of about fifty players or fewer) is called a chamber orchestra.
A full size orchestra (about 100 players) may sometimes be called a "symphony orchestra" or "philharmonic orchestra"; these prefixes do not necessarily indicate any strict difference in either the instrumental constitution or role of the orchestra, but can be useful to distinguish different ensembles based in the same city (for instance, the London Symphony Orchestra and the London Philharmonic Orchestra). A symphony orchestra will usually have over eighty musicians on its roster, in some cases over a hundred, but the actual number of musicians employed in a particular performance may vary according to the work being played and the size of the venue. A leading chamber orchestra might employ as many as fifty musicians; some are much smaller than that.
The typical symphony orchestra consists of four proportionate groups of similar musical instruments, generally appearing in the musical score in the following order:
The Instruments marked in boldface are considered the "core" symphonic instruments, required in all but the rarest cases. Other instruments listed above are considered auxiliary instruments and are less frequently required, but still referred to as standard. Late 19th century symphonic works calling for all the auxiliary instruments, as well as an augmented number of strings, usually include the phrase "for large orchestra" in their full titles.
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, each with its principal. The principal first violin is called the concertmaster (or "leader" in the UK) and is considered the leader of not only the string section, but of the entire orchestra, subordinate only to the conductor.
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. Similarly, the principal oboe is considered the leader of the woodwind section, and is the player to whom all others tune. The horn, while technically a brass instrument, often acts in the role of both woodwind and brass. 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 his or her absence.
A section string player plays 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, for example in the violins, the section leader invariably plays that part. 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, using instead the concertmaster or the harpsichordist playing the continuo for this role. Some modern orchestras also do without conductors, particularly smaller orchestras and those specialising in historically accurate performances of baroque music and earlier.
In the 17th century and early 18th century, instrumental groups were taken from all of the available talent. A composer such as Johann Sebastian Bach had control over almost all of the musical resources of a town, whereas Handel would hire the best musicians available. This placed a premium on being able to rewrite music for whichever singers or musicians were best suited for a performance—Handel produced different versions of the Messiah oratorio almost every year.
As nobility began to build retreats away from towns, they began to hire musicians to form permanent ensembles. Composers such as the young Joseph Haydn would then have a fixed body of instrumentalists to work with. At the same time, travelling virtuoso performers would write concerti that showed off their skills, and they would travel from town to town, arranging concerts along the way. The aristocratic orchestras worked together over long periods, making it possible for ensemble playing to improve with practice.
Throughout the late 18th century composers would continue to have to assemble musicians for a performance, often called an "Academy", which would, naturally, feature their own compositions. In 1781, however, the Leipzig Gewandhaus Orchestra was organized from the merchants concert society, and it began a trend towards the formation of civic orchestras that would accelerate into the 19th century. In 1815, Boston's Handel and Haydn Society was founded, in 1842 the New York Philharmonic and the Vienna Philharmonic were formed, and in 1858, the Hallé Orchestra was formed in Manchester. There had long been standing bodies of musicians around operas, but not for concert music: this situation changed in the early 19th century as part of the increasing emphasis in the composition of symphonies and other purely instrumental forms. This was encouraged by composer critics such as E.T.A. Hoffmann who declared that instrumental music was the "purest form" of music. The creation of standing orchestras also resulted in a professional framework where musicians could rehearse and perform the same works repeatedly, leading to the concept of a repertoire in instrumental music.
The effect of the invention of valves for the brass was felt almost immediately: instrument-makers throughout Europe strove together to foster the use of these newly refined instruments and continuing their perfection; and the orchestra was before long enriched by a new family of valved instruments, variously known as tubas, or euphoniums and bombardons, having a chromatic scale and a full sonorous tone of great beauty and immense volume, forming a magnificent bass. This also made possible a more uniform playing of notes or intonation, which would lead to a more and more "smooth" orchestral sound that would peak in the 1950s with Eugene Ormandy and The Philadelphia Orchestra and the conducting of Herbert von Karajan with The Berlin Philharmonic.
During this transition period, which gradually eased the performance of more demanding "natural" brass writing, many composers (notably Wagner and Berlioz) still notated brass parts for the older "natural" instruments. This practice made it possible for players still using natural horns, for instance, to perform from the same parts as those now playing valved instruments. However, over time, use of the valved instruments became standard, indeed universal, until the revival of older instruments in the contemporary movement towards authentic performance (sometimes known as "historically informed performance").
At the time of the invention of the valved brass, the pit orchestra of most operetta composers seems to have been modest. An example is Sullivan's use of two flutes, one oboe, two clarinets, one bassoon, two horns, two cornets (a piston), two trombones, drums and strings.
During this time of invention, winds and brass were expanded, and had an increasingly easy time playing in tune with each other: particularly the ability for composers to score for large masses of wind and brass that previously had been impractical. Works such as the Requiem of Hector Berlioz would have been impossible to perform just a few decades earlier, with its demanding writing for twenty woodwinds, as well as four gigantic brass ensembles each including around four trumpets, four trombones, and two tubas.
With the advent of the early music movement, orchestras where players worked on execution of works in styles derived from the study of older treatises on playing became common. These include the London Classical Players under the direction of Sir Roger Norrington and the Academy of Ancient Music under Christopher Hogwood, among others.
A special kind of orchestra is the Frankfurter Seniorenorchester directed by Rail Grodzenski.
Some ensembles, such as the Orpheus Chamber Orchestra, based in New York City, have had more success, although decisions are likely to be deferred to some sense of leadership within the ensemble (for example, the principal wind and string players).
Others have returned to the tradition of a principal player, usually a violinist, being the artistic director and running rehearsals (such as the Australian Chamber Orchestra and the New Century Chamber Orchestra).
The techniques of polystylism and polytempo music have recently led a few composers to write music where multiple orchestras perform simultaneously. These trends have brought about the phenomenon of polyconductor music, wherein separate sub-conductors conduct each group of musicians. Usually, one principal conductor conducts the sub-conductors, thereby shaping the overall performance. Some pieces are enormously complex in this regard, such as Evgeni Kostitsyn's Third Symphony, which calls for nine conductors.
Charles Ives used two conductors, one to simulate a marching band coming through his piece. Realizations for Symphonic Band includes one example from Ives.
In some theaters, the orchestra is the area of seats directly in front of the stage (called primafila or platea); the term more properly applies to the place in a theatre, or concert hall reserved for the musicians.