An orchestra is a type of instrumental music ensemble that has been in use since the seventeenth century. Orchestras are classified according to size, with a chamber orchestra being a small group of about 50 musicians, while the term symphony orchestra or philharmonic refers to a larger group of about 100.

Instruments have been used to accompany dance music since ancient times, but the orchestra as we know it today developed in the 17^th century as part of the emerging theatrical style known as opera. It soon evolved into an ensemble worthy of its own repertoire (or body of music) without singers.

Orchestras may include a wide variety of instruments, but the string family makes up the largest portion of an orchestra. Standard orchestral instrumentation has evolved over the centuries – Baroque composers like Vivaldi wrote for a very different sort of orchestra from the ones we have today.

Baroque	Classical	Romantic	Modern
12 violins 4 violas 2 cellos 1 bass	12 violins 4 violas 3 cellos 2 basses	32 violins 12 violas 10 cellos 8 basses	32 violins 14 violas 12 cellos 10 basses 1-2 harps
2 flutes 2 oboes 2 bassoons	2 flutes 2 oboes 2 clarinets 2 bassoons	3 flutes/piccolo 3 oboes/English horn 3 clarinets/bass clarinet 3 bassoons	3 flutes/piccolo 3 oboes/English horn 3 clarinets/bass clarinet 4 saxophones 3 bassoons
2 horns 2 trumpets	2-4 horns 2 trumpets	4-8 horns 4 trumpets 3 trombones 2 tubas	4-8 horns 5 trumpets 6 trombones tuba
harpsichord	clavier	piano celesta	piano celesta
2 timpani	2 timpani	4 timpani snare bass cymbals tam-tam triangle tambourine glockenspiel xylophone chimes	timpani snare bass cymbals tam-tam wood block tambourine glockenspiel xylophone vibraphone chimes marimba

An orchestra is typically led by a conductor. It is the job of the conductor to interpret the composer’s intent for the music. (S)he achieves this by reading the printed music, called the score, and by researching the song’s history. The conductor guides the orchestra in their preparation of the music so that they can deliver the best possible performance.

There are also leaders within each section of the orchestra. These players are called principals. Orchestral musicians are seated in order according to their skill, with principals holding the first chair position. The principal violinist holds the additional title of concertmaster. It is the concertmaster’s responsibility to determine bowing patterns for the string section so that all of their bows move up and down in unison. The concertmaster also tunes the orchestra before each performance.