Many of you woodworkers might have a workbench like mine.  My end vise has a single screw and rides on two pipe pieces.  It is a simple mechanism but when clamping anything on the corner of the vise the jaws won't remain parallel.  This idea came to me to help keep the jaws aligned when I'm using just the corner of the vise.

I made a stack of 1/4 x 2 x 3 inch pieces and used a carraige bolt and wing nut to secure them.  I then put the number of blocks in the opposite end of the vise that matches the thickness of the board I'm trying to clamp. 

  

 

Now the jaws of the vise are parallel and will hold the work solidly.