"You can absolutely cut a stair stringer if only you only knew the way the math works," my dad once said to me. So, long before I ever had a geometry class, I knew the math behind determining a slope.

Amateur carpenters who have laid out the cuts in a stair stringer (the zig-zag structural support for the steps) know the queasy feeling that bubbles up at the prospect of this job. There’s the simple ...