Why is a chamber ensemble of two violins, viola and cello the most popular in all of music? The string quartet has inspired - and instilled fear into - composers like no other ensemble, and has been used in pop songs from The Beatles to Bjork. Tom Service explores the string quartet, from Haydn's epic 68 works for the medium, to Beethoven's heroic and tortured late masterpieces, to Shostakovich's 15 soul-bearing 20th-century works. Tom's guests are composer Dobrinka Tabakova, who takes inspiration from the wealth of quartets written before her, and one of the best quartets in the business - the Brodsky Quartet who, besides the great classical cannon, have played with pop artists including Elvis Costello, Sting and Paul McCartney in their nearly 50-year existence.