NB Edited to extend some points.
Made the mistake of reading some of the comments under that Youtube video.... Some people really don't understand their musical history....
Quick summary: a group of fretted stringed instruments loosely tuned in 4ths (with the odd 3rd) came into Europe via Spain probably from North Africa during the Crusades. In Spain they were referred to as 'vihuela'. Originally they were plucked and known as the vihuela da mano. By around 1400 the plucked version was becoming the guitar and a bowed version evolved which was played with a bow and either sat on the lap or between the legs for the larger ones. This gets the Italian name 'viola da gamba' with 'viola' being the Italian derivative from 'vihuela' and 'da gamba' because it was played 'on the leg'. The violin is 'da braccio' or 'on the arm'.
Viol is the English name and viols were very popular instruments played at home in a group or consort, or with other instruments (as in a 'broken consort'). It was common for wealthier families to have a 'chest' of viols - literally a large box containing 6 viols - 2 trebles, 2 tenors, 2 basses. There is a 'double bass' viol: that is in fact the ancestor of the modern double bass - which is why a)it has sloping shoulders, and b)it is tuned in 4ths not 5ths. If a double bass has 'square' shoulders, then it is a bass violin. There are various other sizes of viol including a higher pitched pardessus and an alto. The baroque era standardised on treble, tenor and bass. The treble and bass were tuned DGCEAD an octave apart, and the tenor GCFADG a 4th above the bass (pardessus an octave above that and the violine an octave below).
Unlike the violin family, the viols are properly sized for their pitch, which is why the tenor viol is the size of a guitar and not a viola. It sounds a 4th lower (low G) than a viola (C in the 2nd space bass clef) but the viola is woefully under-size for its pitch which is wy it sounds congested. The standard bass viol is about the size of a cello and goes to the D below bass clef (cello is a tone lower at C). The treble is one octave higher than the bass. The violine is one octave lower than the tenor.
French bass viols often had a 7th string - low A - and virtuosic bass solo music was a feature of French viol music.
The bass has always been the solo member of the family and English composers such as Christopher Simpson also wrote virtuoso music for bass.
There is a smaller bass viol called the Lyra Viol. It is designed to be able to make it easier to play chords and is typically played from music notated in lute tablature rather than staff notation. I have some...
