I bought mine to hold shop-made jigs in place while routing and sawing.  They are very convenient to use.  The jaw has a maximum opening of about 2", but to get a really tight grip, about an 1 1/2" thick or less works best.  I always keep the black release lever on the top handle if I can.  Two of these will hold a jig very tightly.  The rubber pads will not mar your workpiece or slip.