Name of tool used to check/make squareness

I urgently need to know the name of this tool.
I'm not talking about a try square.

It has for pieces of plastic which have inside corners in them, exactly 90 degrees, a piece of string goes around each plastic corner and you put the the work that you want to be rectangular (all 4 outside corners 90 degrees) inside the corner pieces of the tool, and pull the string to make it very tight.

You can glue the wood together that you are making, put this tool around it and tie off the string, the woodwork will then glue in a perfect rectangle, with every corner at 90 degrees.