Make sure the post is 'square' to the floor. [=make sure that there is a right angle where the post and the floor meet]
I'll go on the trip if I can 'square it with' my parents. [=if my parents will agree to let me go]
Here's the rest of the money. Now we're 'square'. [=now we don't owe each other anything]
(..) Lieutenant Kaffee, that's not in the book, sir. You mean to say in all your time at Gitmo you've never had a meal? No, sir. Three squares a day, sir. I don't understand. How did you know where the mess hall was if it's not in this book? Well, I guess I just followed the crowd at chow time, sir. (..)1992 A Few Good Men