Though I’m interested in all types of viols, I’m focused on late 16th through early 17th century English and French instruments made with "bent fronts" or tops.
These were made using the technique of bending and then joining 5 or 7 thin strips of spruce to create the complex shape of the arched front. This is unlike other types of viols and violin family instruments which have fronts made from two thick wedges joined in the center and then carved to shape.
This type of construction was only identified with certainty in surviving instruments about 40-50 years ago but is now well documented. One can speculate, but nothing was written down as to tell us exactly why this method of construction was used. Bending and assembling strips of bent wood was a fairly common, highly skilled technique used in those days, barrel and ship making being obvious examples and was already in use in the construction of other instruments, such as the lute.
Was it done because most of the larger trees had been cut down and wood wide enough for two piece tops was hard to come by, or were the makers of these instruments so skilled at the art of bending and joining pieces of wood that it was less work than fully carving from thick wedges? Or was it to economize, where 2 or even 3 bent fronts could be made from the same amount of wood needed for a single fully carved front?
Whatever the reason, from a structural point of view, this is a very significant feature. The wood fibers of the front follow through the complex, curving shapes, almost as if it had grown that way.
The natural virtue of the design allows for thinner and lighter tops than the carved version, yet making durable, resilient tops that are resonant and responsive ...and that ultimately have their own characteristic sound. Not better than carved, but different and quite appropriate for reproducing the sounds of the time.
There are many elements that combine to make a successful instrument and all of those must be done well, the "setup" including the right strings types and gauges. But It was a design that must have been well appreciated, for whatever reason, as it was used by all the best known makers of it's day.
Once the top pieces are joined and the final shape and thickness is achieved, strips of linen or vellum were used inside the instrument to reinforce all joints.