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]
(..) You haven't spent much time around cemeteries, have you? Mr. Thorwald could scarcely put his wife's body... in a plot of ground about one foot square. Unless, of course, he put her in standing on end. Then he wouldn't need a knife and saw. No, my idea is she's (..)1954 Rear Window