Bass viol on the left, cello on the right

Despite their similarity in size and shape, the bass viol and the cello belong to different families. The bass viol is one of several sizes that form the viola da gamba family, while the cello is the bass member of the violin family, more formally known as the viola da braccio family, literally 'arm fiddles.' Even though the cello is held with the legs, i.e. 'da gamba,' it is really a large violin. Here are some of the chief differences distinguishing the two families:

Instruments in the viola da gamba family usually have six or seven strings which are tuned in fourths with a third in the middle.

Instruments in the violin family usually have four strings which are tuned in fifths.

Viol players hold the bow underhand with the palm facing up while the bows of violin family instruments are held overhand with the palm facing down

Sometimes viols even have a third 'rosette' sound hole between the bridge and the fingerboard

Viols have C-shaped sound holes while those of violins and cellos are F-shaped

Viols usually have a flat back with a bend near the top while violins have curved backs

Viols usually have square corners and top and back plates that don't overhang the ribs

Violins and cellos have pointed corners and top and back plates that do overhang the ribs

Violin family instruments have relatively shallow ribs and thin bodies while viols have deeper ribs and thicker bodies

Viols are also usually more highly decorated than cellos including elaborately carved heads instead of scrolls

While the above guidelines are the norms, there are many exceptions. During the 18th- and 19th-centuries, many viols were converted into cellos and others were constructed in a somewhat hybrid form including elements of both family types. Large bass instruments were especially variable and the modern double bass, a member of the viola da braccio family, retains some characteristics of the viola da gamba family, such as being tuned in fourths.
 

Both families were built in 'consorts,' that is, matched sets of instruments of different sizes, roughly equivalent to the different human voice ranges. The viols emerged in the fifteenth century, while the violins made their first appearance in the 1520s. They were independent families, neither one descended from the other. The louder violin family instruments were often viewed as the less refined cousins of the noble viols, the former better suited to dance music and the latter to the quieter social music making that was cultivated by amateurs and professionals at courts and domestic settings.