Wikipedia source excerpts
The string section, including the violin, viola, cello, and double bass
The woodwind section, including the flute, oboe, clarinet, and bassoon, and sometimes piccolo, cor anglais, bass clarinet, contrabassoon, and saxophone
The brass section, including the French horn (commonly known as the "horn"), trumpet, trombone, and tuba, and sometimes cornet and euphonium
The percussion section, including the timpani, snare drum, bass drum, cymbals, triangle, tambourine, tam-tam and mallet percussion instruments
The harp is fairly regularly included in modern orchestras, especially for romantic period music. Keyboard instruments such as the piano, harpsichord, pipe organ, free-bass accordion and celesta may sometimes appear in a fifth keyboard section or may stand alone as soloist instruments. For performances of some modern compositions, electronic instruments and guitars may also be included.
A full-size Western orchestra may sometimes be called a symphony orchestra or philharmonic orchestra (from Greek phil-, "loving", and "harmony"). The number of musicians employed in a given performance may vary from seventy to over one hundred, depending on the work being played and the venue size. A chamber orchestra (sometimes a concert orchestra) is a smaller ensemble of not more than around fifty musicians. Orchestras that specialize in the Baroque music of, for example, Johann Sebastian Bach and George Frideric Handel, or Classical repertoire, such as that of Haydn and Mozart, tend to be smaller than orchestras performing a Romantic music repertoire such as the symphonies of Ludwig van Beethoven and Johannes Brahms. The typical orchestra grew in size throughout the 18th and 19th centuries, reaching a peak with the large orchestras of as many as 120 players called for in the works of Richard Wagner and later Gustav Mahler.
Orchestras are usually led by a conductor who directs the performance with movements of the hands and arms, often made easier for the musicians to see by using a short wooden rod known as a conductor's baton. The conductor unifies the orchestra, sets the tempo, and shapes the sound of the ensemble. The conductor also prepares the orchestra by leading rehearsals before the public concert, in which the conductor provides instructions to the musicians on their interpretation of the music being performed.
The leader of the first violin section – commonly called the concertmaster – also plays an important role in leading the musicians.
Contemporary music
The techniques of polystylism and polytempo music have led a few 20th and 21st century composers to write music where multiple orchestras or ensembles 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. In Percy Grainger's The Warriors which includes three conductors: the primary conductor of the orchestra, a secondary conductor directing an off-stage brass ensemble, and a tertiary conductor directing percussion and harp. One example in the late-century orchestral music is Karlheinz Stockhausen's Gruppen, for three orchestras, which are placed around the audience. This way, the "sound masses" could be spatialized, as in an electroacoustic work. Gruppen was premiered in Cologne, in 1958, conducted by Stockhausen, Bruno Maderna and Pierre Boulez.
Expanded instrumentation
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 Boléro, Sergei Prokofiev's Romeo and Juliet Suites 1 and 2, Vaughan Williams' Symphonies No. 6 and No. 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, Igor Stravinsky (as featured in The Rite of Spring), Béla Bartók, and others; it also has a notably 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,
ondes martenot, or trautonium, as well as other non-Western instruments, or other instruments not traditionally used in orchestras including the: bandoneon, free bass accordion, harmonica, jews harp, mandola and water percussion.
With this history in mind, the orchestra can be analysed in five eras: the Baroque era, the Classical era, early/mid-Romantic music era, late-Romantic era and combined Modern/Postmodern eras. The first is a Baroque orchestra (i.e., J.S.