I recently finished making a miter jack, following the tutorial and plans posted at the Benchcrafted web site. After using it for a couple of weeks I've found it indispensable for paring the various surfaces on the three-way corner joint described in one of Chris's monographs. Perhaps others would find it useful, too.
After doing a bit of research I've found a few different miter jack designs. All can do 45 degree angles, most can do 45's and 90's, and this French design I've copied will also do 22.5 degree angles. I suppose one could make one that would do 30/60 degrees in the same way.
Mine is walnut. I adapted it to mount between bench dogs. Other designs clamp between vise jaws or are held by holdfasts.
The key to using it efficiently is to make sure the 90 and 45 degree surfaces are dead accurate, level, and flat since they are the reference surfaces for paring or planing. I had access to a vernier protractor to check within 12 minutes of a degree but other tools or methods might work as well.
One plus over shop made paring blocks or jigs is that it has an integral screw and vise jaw. With shop made jigs that rely on clamps I sometimes feel like I need a third hand to get everything set up properly. And, I think it lovely, if I do say so. The metal bits came from Benchcrafted. I made the screw using a Beall system threader and tap.
Set up for 45 degrees.
Flipped over for 90 degrees
Paring a miter:
Paring into a blind pocket:
And for plain ol' miters, too. This test miter was planed rather than pared. Both halves planed at the same time, then assembled right off the miter jack. That is a 6" square.
I am very happy I made this. I see myself using it a lot.