To draw perfect, smooth, uninterrupted curved objects, woodworkers use heavy lead weights and a thin strip of clear grained wood. They bend the strip to shape, and then hold it in place with these precision weights.
The exact dimensions of the wood aren't significant, although it must be consistent in thickness and width, and bend easily to the desired curves, for each curve radius a different thickness is used.
Ed Claxton, a guitar maker working in Santa Cruz, still uses this drafting method that came from craftspeople who design or build irregularly curved objects such as boats, planes, cars, or even complex roofs. In fact any objects that has to play nicely with flows.
They go by various names depending on who is using them. In cars, the curves are called ‘spline’ and weights are ‘ducks’; roofers who lay out ‘eyebrow’ rafters for shingled roofs simply use a straight strip of plywood and a few nails.
[ ... ] just like magic, the wood strip springs out or in just a teeny bit to smooth out the curve. A couple of trips around the curve, and it comes out perfectly aligned. [...] this is a trick that works so well, it is really not possible to describe adequately. You really should see it in action. Working a section at a time, he gets each curve just right. Of course, it's no trick at all to [then] draw a pencil line along this strip.
This is an all craft practice. A large range of manual tools surround Claxton in his instrument making, yet when it comes to designing, he employs this age-old technique to produce perfectly smooth curves. He knows that for this specific task he'll never be able to outdo this technique. Claxton even describes the results in terms of magic.
It's quite curious how fast this technique, that was obviously perfectly adapted for design, was discarded. drafting and adjusting balanced curves is a complex task that is hard to get guidance for in the software realm. Relying on naturally available tensions when it comes to the smoothness of curves seems to be something we should revisit.
Quotes and pictures from http://www.frets.com/FretsPages/Luthier/TipsTricks/DraftingWeights/draftweights.html