I spent a bit working at a custom stairbuilder. Handrails that smoothly curve in three dimensions, either to connect straight sections or along a curved stair,

I spent a bit working at a custom stairbuilder. Handrails that smoothly curve in three dimensions, either to connect straight sections or along a curved stair, are called "tangent" handrails. Constructing these tangent handrails is challenging. It requires both geometrical rigor in devising the plans as well as an individual craftsperson with a very high level of manual/visual skill. While we would work out the curvature of the rail in CAD or paper plans, a huge amount of implementation detail was left to a traditional woodworker, such as carving the profile of the tangent rail to create transitions pleasing to the eye and touch. Additionally, the variable structure of wood grain meant that the choice and orientation of the wood stock often had to be done piece-by-piece, often with custom fixtures. I was a resident computer nerd there, so one of my projects was adapting a 5-axis CNC router to rough out these tangent rails. We were successful to some degree, but the fixturing and programming for individual tangent sections was very complex. It worked out that the CNC approach was only economical when the design of the stair required many multiples (rare in our case). Otherwise the skill and adaptability of individual craftsman was much more efficient, and the CNC was relegated to constructing jigs or 2D pieces. My training was in high-end artisan furniture making, and the design challenges I saw in custom stairs were way more complex than almost any other field of woodworking I am familiar with, including the more technically avant-garde furniture makers. The only clear exception is wooden boat building, which appropriately enough was the background of many of my coworkers at that company.