Violone, It. (large
viola).
The lowest bowed string instrument in the Baroque. Ancestor of the modern double bass.
Unlike what Webster 1913 claims, the double bass and the violone are not quite the same. The double bass is a descendant of the violone. The violone was used mostly in the 18th century, whereas the double bass as we know it was only starting to appear at that time. The term violone is not as precise as most people think; it simply means large viola (da gamba) or viol. This was applied to any form of stringed bass instrument, where the size, shape, strings, tuning and origin were often not important and not specified. Usually it was supposed to sound in the register of the modern double bass, but this was seldom specified, just implied.
In the Viennese tradition, the Violone had five strings (although four and six could be found), often tuned to something like A-F#-D-A-F# (top to bottom), thus making the key of D Major very easy and most others unnecessarily hard. This actually led to a virtuoso school in Vienna and parts of Germany in the mid 18th century, a school that didn't survive very long. No composer would want musicians who can only play in one key!
There were other types of violones as well, very varied. Generally, a violone is somewhat smaller than the modern double bass, is tuned differently, MAY have frets at the lowest positions, a shorter fingerboard (no high-register solos were imagined at the time) and looks more like a Baroque instrument (which it really is).
The violone survived in Vienna and Germany for some time after the rest of Europe had moved to the double bass (tuned mostly in fourths at the time except for Italy and England, nowadays almost exclusively so). The fourth tuning was found to be the most playable in terms of shifting (intonation, accuracy and speed), and also harmonically. Thus, the double bass soon took over the market.
The statement "having strings tuned an octave below those of the violoncello" is, unfortunately, imprecise. The strings of a violone were never tuned the same way as a cello (which is tuned A-D-G-C top to bottom). The violone does encompass (parts of) the octave below that of the cello, but has always been tuned differently. Today, some double bassists tune their instruments excactly an octave below the cello, but these are extremely rare...