An orchestra is a large instrumental ensemble containing string sections (violin, viola, cello, and double bass), brass, woodwind, and percussion instruments. Other instruments such as the piano and celesta may sometimes be grouped into a fifth section such as a keyboard section or may stand alone, as may the concert harp and electric and electronic instruments. The orchestra grew by accretion throughout the eighteenth and nineteenth centuries but changed very little in composition during the twentieth century.
A smaller-sized orchestra for this period (of about fifty musicians or fewer) is called a chamber orchestra. A full-size orchestra (about one hundred musicians) may sometimes be called a symphony orchestra or philharmonic orchestra; these modifiers do not necessarily indicate any strict difference in the orchestra's instrumental constitution or role but can be useful to distinguish different ensembles based on the same city. 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 term concert orchestra may sometimes be used no distinction is made on the size of orchestra by use of this term, although their use is generally distinguished as for living concert. As such they are commonly chamber orchestras. There are several types of amateur orchestras, including school orchestras, youth orchestras, and community orchestras.
Orchestras are usually led by a conductor who directs the performance by way of visible gestures. The conductor unifies the orchestra, sets the tempo, and shapes the sound of the ensemble.
The typical symphony orchestra consists of four related musical instruments: the woodwinds, brass, percussion, and strings (violin, viola, cello, and double bass). Other instruments such as the piano and celesta may sometimes be grouped into a fifth section such as a keyboard section or may stand alone, as may the concert harp and electric and electronic instruments. The orchestra, depending on the size, contains almost all of the standard instruments in each group. In the history of the orchestra, its instrumentation has been expanded over time, often agreed to have been standardized by the classical period and Ludwig van Beethoven’s influence on the classical model. In the twentieth century, new repertory demands expanded the instrumentation of the orchestra, resulting in the flexible use of the classical model instruments in various combinations.
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, the first violin, and the second violin, with the second violin playing with lower registers than the first violin.
The principal first violin is called the concertmaster 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 all of the string sections. The concertmaster usually sits to the conductor’s left, closest to the audience. In some U.S. and British orchestras, the concertmaster comes on stage after the rest of the orchestra is seated, takes a bow, and receives applause before the conductor appears on stage.
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 three-hundred-year-old convention), no principal is the leader of the woodwind section though in woodwind ensembles, often the flute is a leader. Instead, each principal confers with the others as equals in the case of musical differences of opinion. 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 (all together) part in addition to replacing the principal in his or her 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. 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.
Conducting is the art of directing a musical performance, such as a concert, by way of visible gestures with the hands, arms, face, and head. The primary duties of the conductor are to unify performers, set the tempo, execute clear preparations and beats, listen critically and shape the sound of the ensemble.