Rack and Rule

With a traditional single screw front vice, putting a workpiece in one end causes the moving jaw to rack diagonally and pinch one side of the workpiece without touching the other.

In some circumstances (holding a tapered leg for example) racking can be quite helpful but if you do want the jaws to close square you can eliminate racking easily enough by putting something of a similar thickness in the other end of the vice.

That said, rummaging around for a similar thickness something isn't exactly conducive to efficient workflow and you then need to hold the workpiece, the something and operate the vice all at the same time.

Enter the humble wooden folding rule in it's capacity as a something of variable thickness for putting in the other end of the vice. Just fold out the appropriate number of leaves and pop it in place, leaving both hands free to position the workpiece and operate the vice.