A perfect circle may indeed be “unattainable” according to mathematicians, but the perfect square is within reach — if you have the right clamps. Pinnacle’s clamping square and wedge are just the answer for the frame that needs to be perfect on all corners.
The system consists of a counter-bored square that can be mounted to any flat surface via screws and a wedge that pairs up with the square to create a fixture for forming miter joints. To adjust and tighten the box, attach the wedge to a threaded stud on a toggle clamp and push it into the frame which creates a stable platform for gluing or nailing.
It makes a good jig system if you find yourself doing the same box over and over again — such as when you’re making picture frames.
A pair will run around $29 on the woodcraft site but don’t forget the toggle clamp; it runs around $7, but wedge is pretty useless if you don’t have one.