When gluing up a frameless cabinet, we all know that we have to square up the carcase before the glue cures – otherwise the doors and drawers will never fit well.
When working with beginners and many intermediate woodworkers, they usually square up only the front of the case with diagonal clamping pressure. You can get away with this about half the time – either the diagonal pressure at the front will shift the back of the cabinet into square or the out-of-square back doesn’t affect the final fit of the doors.
But if you are putting drawers into that carcase, a twisted box will make you crazy when you fit them.
That’s why I always check the back or bottom of a carcase to make sure it is just as square as the front. Usually the back needs the same diagonal pressure as the front. But sometimes the case is twisted, and so you have clamps going in opposite diagonals.
In the carcase above, the back is captured in a groove, so I made dead sure the back was square before sliding it in place during assembly. This kept the rear of the carcase square. and I had to apply just a little diagonal pressure at the front of the carcase to get it square.
One final tip: Though it’s good practice to check for squareness by measuring the diagonals in a carcase, I also recommend you follow up by checking all the drawer or door openings with a try square. This confirms you did what you set out to do. Also, if the blade of your square shows the wood has bowed under clamping pressure, something is wrong. Ease off on the clamps.
— Christopher Schwarz