The Croix de St. Pierre, from page 211 of “The Amateur Carpenter and Builder” from the early 20th century. Image courtesy of Gary Roberts.
One of the biggest complaints about leg vises is having to engage some sort of secondary mechanism to keep the jaw parallel as you advance it and to act as a pivot point when squeezing the work. I have a bar in my tail vise that is bored with a series of holes. By moving a steel pin into the correct hole I can control the parallelism of the jaw and set the jaw for different thicknesses. I’m so used to it that I don’t think much about it and it has become part of the natural rhythm of my work.
However, if you don’t like stooping, you won’t like having to do this.
One solution, which is presumably French, is called “Croix de St. Pierre.” I’ve seen it in action on a commercial leg vise and it is ingenious. It is, essentially, two flat pieces of steel that are joined by a hinge in the center, much like scissors, forming an “X” shape. At the top of the X, one end is attached to the bench; the other to the jaw of the vise. The two ends at the bottom run in grooves in the jaw and leg of the bench. The scissors action of the X keeps the jaws parallel as you work.
I’ve always meant to make one of these devices myself for a leg vise, but I’ve always been satisfied with the occasional stoop to move the steel pin.