Let's try this.
This is a drawing of the base plate which a router is mounted. The bit is at the center, in the hole.
The 2 pins, (or bearings) are against either side of the board that you wish to create a mortise. They are on the same center-line as the bit, and equal distance from center. This puts the bit at the center of the board edge, and when you plunge the bit into the wood and slide the base plate along the edge, you create a mortise. That mortise is centered in the board edge.
The jig Bob and I have posted is nothing more than a way to hold the wood steady, and the bit perpendicular, as you create the mortise.
Last edited by AxlMyk; 11-28-2011 at 11:10 PM.