When I worked as a shipwright in Maine, I occasionally had to drill holes through a board from one side and exit in the middle of the other side. Drill bits tend to wander, and it’s quite easy to break out through the surface, which, of course, ruins the work. One day I had to drill 5/16-inch holes through the edges of two oak boards so they could be fastened with bronze bolts.
One of the older shipwrights saw me fumbling and said, “Want me to show you how we do that, sonny?” I watched as he cut out a jig from a hardwood scrap and clamped it to the work as shown. With this in place, he drew a guideline at right angles to the edge and drilled right through the board, keeping the drill bit flat on the jig. The bit exited within 1/16-inch of the centerline. I was suitably impressed and have used the technique ever since.