The string quintet normally comprises the four instruments of the standard string quartet (2 violins, viola, and cello), plus one additional instrument. This additional instrument may be a viola, cello or double bass. Of these three different combinations, the string quintet with a second viola has the widest repertoire and may be referred to as the standard string quintet. The other two combinations have far fewer important works, though the combination with two cellos includes the string quintet by Schubert, which is widely considered the greatest of all string quintets.