The origins of the string quartet can be traced back to the Baroque trio sonata, in which two solo instruments performed with a continuo section consisting of a bass instrument (such as the cello) and keyboard. A very early example is a four-part sonata for string ensemble by Gregorio Allegri (1582–1652) that might be considered an important prototype string quartet.[citation needed] By the early 18th century, composers were often adding a third soloist; and moreover it became common to omit the keyboard part, letting the cello support the bass line alone. Thus when Alessandro Scarlatti wrote a set of six works entitled "Sonata à Quattro per due Violini, Violetta [viola], e Violoncello senza Cembalo" (Sonata for four instruments: two violins, viola, and cello without harpsichord), this was a natural evolution from existing tradition.[1]
Wyn Jones also suggests another possible source for the string quartet, namely the widespread practice of playing works written for string orchestra, such as divertimenti and serenades, with just four players, one to a part, there being no separate (fifth) contabasso part in string scoring before the 19th century