Transfer biscuit locations accurately from a case side to a shelf end using this custom T-square. Assemble it from medium-density fiberboard (MDF) or melamine-covered particleboard scraps to make pencil marks easy to erase. Mark the biscuit location centerlines on the long leg of the square. To use the square, clamp it to a shelf, as shown above, with the short leg against the front edge, and use the lines to cut biscuit slots on the shelf ends. Then clamp the square to the case side with the short leg against the front edge. Using the same centerlines, cut slots on the inside face of each case side.
—from the WOOD shop