Pierrot ensemble

A Pierrot ensemble is a musical ensemble comprising flute, clarinet, violin, cello and piano, frequently augmented by the addition of a singer or percussionist, and/or by the performers doubling on other woodwind/stringed/keyboard instruments. This ensemble is named after 20th-century composer Arnold Schoenberg’s seminal work "Pierrot Lunaire", which includes the quintet of instruments above with a narrator (usually performed by a soprano).
History.
The quintet of instruments used in "Pierrot Lunaire" became the core ensemble for many contemporary-music ensembles of the twentieth century, such as The Fires of London, who formed in 1965 as "The Pierrot Players" to perform Pierrot Lunaire, and continued to concertize with a varied classical and contemporary repertory. This group (and others like it) began to perform works arranged for these instruments and commission new works especially to take advantage of this ensemble's instrumental colors.
While many professional chamber ensembles (such as string quartets and piano trios) continued to focus on musical literature of the 18th and 19th centuries, the Pierrot ensemble became one of the most prominent chamber ensembles in classical music of the 20th century, and continues to be popular with composers and performers today.
Doublings.
Doublings are often called for in music written for Pierrot ensemble. For example, in Schoenberg's "Pierrot Lunaire", the flutist is asked to play piccolo, the clarinetist is asked to play bass clarinet (as in Earle Brown's "Tracking Pierrot"), or saxophone, and (much more unusually) the violinist is asked to play viola. Other common doublings might include E clarinet (as in Carter's "Triple Duo"), alto flute, or even harpsichord (as in Maxwell Davies's "Eight Songs for a Mad King").
References.
Christopher Dromey, "The Pierrot Ensembles: Chronicle and Catalogue, 1912-2012" (London: Plumbago, 2013).