IKEA Billy drilling jig

The IKEA BILLY is perhaps their best-selling, and definitely their most versatile, bookcase system. For reasons that I fail to grasp, they don't have holes everywhere, so if you want shelves with non-standard spacing, you need to drill them yourself.

The idea behind this jig is to use the existing holes to align it, and provide a guide for the drill bit to drill holes that are perpendicular to the surface. Moreover, you don't want the drill bit going all the way through, so the height of the