Take one string quartet, add a piano, stir and mix. The result is the piano quintet.
But, a composer might ask, how should the piano be used? As a soloist accompanied by the strings? To accompany the strings? Or perhaps both - in which case the challenge is how to juggle the musical components among the five players in some logical and satisfying way.
In this hour, how nine composers worked out their own answers, from Boccherini to Brahms to Shostakovich.