Four Patch Square Up Tool by Deb Tucker's Studio 180 Design 

Four Patches; basic units that seem so simple to make. Just stitch 4 squares together and you’re good to go . . . but anyone who has attempted these units knows that they are notoriously difficult to make perfectly! Either the unit size isn’t quite right, the edges just don’t line up, or they’re just not quite square. Well, the Four Patch Square Up Tool is the solution!
Like all the Studio 180 Design rulers and processes, this tool and technique will have you build your units slightly oversized and then quickly trim them to perfect shapes. Give this tool a try, you'll be absolutely thrilled with the results.
Four Patch Square Up tool available here: bit.ly/FourPatchSquareUp