I nearly always find the shelf support hole spacing on bookcases are inconvenient, being too far apart

I therefore designed this customizable design to allow intermediate holes to be drilled with ease. It is intended to use the original support pegs where long enough or spare drill bits can be used.

One end hole is slotted to allow for a little inaccuracy in hole spacing measurement; make sure the jig is always fitted the same way up to get consistent spacing.