A cool stretcher bar tool

I recently went out of town and left Chris and the dogs in charge of the Bristly Thistle.  On Chris' to do list was putting together 20 sets of stretcher bars, with my special admonition that they be square.  Chris' solution (at around $5) was a T square from the LHS (local hardware store).  You can even use it to see if your stretcher bars are level!