Show a rail to the table leg to determine the amount of reveal you’d like. (Many tables’ rails are not flush to the legs but are recessed a bit.) To envision how far the mortise should be from the leg’s outside face, you can set your mortise chisel to it. Typically, the mortise is approximately centered on the rail’s thickness. Mark the mortise position onto the leg with your knife and set your mortise gauge to scribe the lines. It is common practice to allow the gauge lines to run a little past the bottom mortise line, so don’t bother trying to make a perfect stop there. If you’ve resisted buying a mortise gauge, you really ought to remedy that. You can easily modify an old beater marking gauge – two nails filed to knife edges can be adjusted with pliers to perfectly match your mortise chisel width.
I love using a low “staked” bench for mortising, as did many period chairmakers, because this puts me right on top of the work and in line with the chisel. Often, they sat directly on the workpiece, which freed them from the inconvenience of setting holdfasts. I like the simplicity of this method, but I must not have enough posterior padding because I haven’t found it very comfortable balancing on one little leg. Sitting on all four legs at once isn’t too bad, though. I plan to experiment with these methods further.
When I mortise sitting down like this, I like to start on the end of the mortise closest to me and progressively move backward with the bevel facing toward me. When the chisel is facing the other way, I’m forced to put the mallet into my chest. It’s one of those little things that makes a noticeable difference.
–Joshua A. Klein, excerpt from “The Artisan’s Guide to Pre-industrial Table Construction,” in Issue Four