The earliest cellos were developed during the 16th century and frequently were made with five strings. They served mainly to reinforce the bass line in ensembles. Only during the 17th and 18th centuries did the cello replace the bass viola da gamba as a solo instrument. During the 17th century the combination of cello and harpsichord for basso continuo parts became standard. Joseph Haydn (Haydn, Joseph), Mozart (Mozart, Wolfgang Amadeus), and later composers gave increased prominence to the cello in instrumental ensembles. Notable works for the instrument include J.S. Bach's (Bach, Johann Sebastian) six suites for unaccompanied cello, Beethoven's (Beethoven, Ludwig van) five sonatas for cello and piano, the concertos of Antonín Dvořák (Dvořák, Antonín) and Darius Milhaud (Milhaud, Darius), the sonatas of Zoltán Kodály (Kodály, Zoltán) and Claude Debussy (Debussy, Claude), and the Bachianas brasileiras of Heitor Villa-Lobos (Villa-Lobos, Heitor), for eight cellos and soprano. One of the outstanding cellists of the 20th century was Pablo Casals (Casals, Pablo).