An orchestra ( or US: ; Italian: [or'kestra]) is a large instrumental ensemble typical of classical music, which mixes instruments from different families, including bowed string instruments such as violin, viola, cello and double bass, as well as brass, woodwinds, and percussion instruments, each grouped in sections. Other instruments such as the piano and celesta may sometimes appear in a fifth keyboard section or may stand alone, as may the concert harp and, for performances of some modern compositions, electronic instruments. The term orchestra derives from the Greek orkhestra (orchestra), the name for the area in front of a stage in ancient Greek theatre reserved for the Greek chorus. A full-size orchestra may sometimes be called a symphony orchestra or philharmonic orchestra.

Apart from the core orchestral complement, various other instruments are called for occasionally. These include the flugelhorn and cornet. Saxophones and classical guitars, for example, appear in some 19th- through 21st-century scores. While appearing only as featured solo instruments in some works, for example Maurice Ravel's orchestration of Modest Mussorgsky's Pictures at an Exhibition and Sergei Rachmaninoff's Symphonic Dances, the saxophone is included in other works, such as Ravel's Bolero, Sergei Prokofiev's Romeo and Juliet Suites 1 and 2, Vaughan Williams' Symphonies No.6 and 9 and William Walton's Belshazzar's Feast, and many other works as a member of the orchestral ensemble. The euphonium is featured in a few late Romantic and 20th-century works, usually playing parts marked "tenor tuba", including Gustav Holst's The Planets, and Richard Strauss's Ein Heldenleben. The Wagner tuba, a modified member of the horn family, appears in Richard Wagner's cycle Der Ring des Nibelungen and several other works by Strauss, Bela Bartok, and others; it has a prominent role in Anton Bruckner's Symphony No. 7 in E Major. Cornets appear in Pyotr Ilyich Tchaikovsky's ballet Swan Lake, Claude Debussy's La Mer, and several orchestral works by Hector Berlioz. Unless these instruments are played by members "doubling" on another instrument (for example, a trombone player changing to euphonium or a bassoon player switching to contrabassoon for a certain passage), orchestras typically hire freelance musicians to augment their regular ensemble.  The 20th-century orchestra was far more flexible than its predecessors. In Beethoven's and Felix Mendelssohn's time, the orchestra was composed of a fairly standard core of instruments, which was very rarely modified by composers. As time progressed, and as the Romantic period saw changes in accepted modification with composers such as Berlioz and Mahler; some composers used multiple harps and sound effect such as the wind machine. During the 20th century, the modern orchestra was generally standardized with the modern instrumentation listed below. Nevertheless, by the mid- to late 20th century, with the development of contemporary classical music, instrumentation could practically be hand-picked by the composer (e.g., to add electric instruments such as electric guitar, electronic instruments such as synthesizers, non-Western instruments, or other instruments not traditionally used in orchestra).  With this history in mind, the orchestra can be analyzed in five periods: the Baroque era, the Classical music period, early/mid-Romantic music era, late-Romantic/early 20th century music and 21st century era. The first is a Baroque orchestra (i.e., J.S. Bach, Handel, Vivaldi), which generally had a smaller number of performers, and in which one or more chord-playing instruments, the basso continuo group (e.g., harpsichord or pipe organ and assorted bass instruments to perform the bassline), played an important role; the second is a typical classical period orchestra (e.g., early Beethoven along with Mozart and Haydn), which used a smaller group of performers than a Romantic music orchestra and a fairly standardized instrumentation; the third is typical of an early/mid-Romantic era (e.g., Schubert, Berlioz, Schumann); the fourth is a late-Romantic/early 20th century orchestra (e.g., Wagner, Brahms, Mahler, Stravinsky), to the common complement of a 2010-era modern orchestra (e.g., Adams, Barber, Aaron Copland, Glass, Penderecki).

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