A string quartet is a piece of music for four string instruments.A string quartet can also mean the four people who play a piece for four string instruments.The four instruments in a string quartet are almost always 2 violins, 1 viola and 1 cello. The reason that a double bass is not used is that it would sound too loud and heavy. The balance between 2 violins, viola and cello is considered ideal.String quartets are the most popular form of chamber music. Many composers have written string quartets.