The double bass is generally regarded as a modern descendant of the string family of instruments that originated in Europe in the 15th century, and as such has been described as a bass Violin. Before the 20th century many double basses had only three strings, in contrast to the five to six strings typical of instruments in the string family or the four strings of instruments in the violin family. Some existing instruments, such as those by Gasparo da Salò, were converted from 16th-century six-string contrabass
The double bass's proportions are dissimilar to those of the violin and cello; for example, it is deeper (the distance from top to back is proportionally much greater than the violin). In addition, while the violin has bulging shoulders, most double basses have shoulders carved with a more acute slope, like members of the viol family. Many very old double basses have had their shoulders cut or sloped to aid playing with modern techniques. Before these modifications, the design of their shoulders was closer to instruments of the violin family.
The double bass is the only modern bowed string instrument that is tuned in fourths (like a viol), rather than fifths The issue of the instrument's exact lineage is still a matter of some debate, and the supposition that the double bass is a direct descendant of the viol family is one that has not been entirely resolved.
In his A New History of the Double Bass, Paul Brun asserts, with many references, that the double bass has origins as the true bass of the violin family. He states that, while the exterior of the double bass may resemble the viola da gamba, the internal construction of the double bass is nearly identical to instruments in the violin family, and very different from the internal structure of viols.