One major difference between Baroque music and the classical era that followed it is that the types of instruments used in ensembles were much less standardized. Whereas a classical era string quartet consists almost exclusively of two violins, a viola and a cello, a Baroque group accompanying a soloist or opera could include one of several different types of keyboard instruments (e.g., pipe organ, harpsichord, or clavichord), additional stringed chordal instruments (e.g., a lute) and an unspecified number of bass instruments performing the basso continuo bassline, including bowed strings, woodwinds and brass instruments (e.g., a cello, contrabass, viol, bassoon, serpent, etc.).