One of the differences you immediately see between instruments belonging to Viol family and those belonging to Violin family is that Viols have fretted fingerboard, this feature have a great influence not only on the playability of the instrument and on the “feeling”, but also on the sound itself, not to talk about the influences on the fine tuning of the action of the strings.
Unlike modern plucked instruments (like guitars), viols’ frets are movable and removable. You can adapt their position to the string length of you viol or the temperament you prefer, making it easy to apply tone corrections if a note is not perfectly in tune.
Frets are generally (and traditionally) made of Gut, usually of a quality, a processing method, and twisting different than the one of the regular strings (luckily making them slightly cheaper). Alternatives to gut frets are possible: Aquila corde produces synthetic frets, some musician use also frets made of fishing nylon, but it’s also possible to use old gut strings. I usually prefer gut frets, as synthetic matters (nylon, amongst all) are harder than gut, and it can result in premature consumption of strings.
Choosing fret gauges
Choosing the right fret gauges is very important and can affect your instrument playability: frets too thick can influence note intonation and can cause string buzzes.
String action on viols ca be very different than the one on Cellos, strings are generally closer to the fingerboard, due to the presence of the frets, the string vibrates differently on the nut and in the middle of its length, so frets must be graduated decreasing the gauges as long as the distance from the nut is longer.
Considering for example a bass viol, the first three frets have thicker gauges, the following two decreased gauges, the next two gauges again decreased. I normally use the gauges of the three top strings, ex. 120 mm, 0.90 mm, 0.72 mm
As gut is a matter prone to consumption (due to rubbing of strings and fingers) and deformations (due to humidity changes), frets as easy to break or to become loose, you can have them changed by a luthier, but you can also do it yourself, it is not a difficult task, if you are good in working with your hands.
Coming soon: a tutorial on how to put and tie knots on viols