I purchased this set of mounting brackets to help me assemble (2) bookshelves for a customer. As some people have complained, they aren't "perfectly" straight/square. If you need this straightness you should buy/use a machine square. Of the (4) I received only one was slightly out of order - maybe 1/64. To keep the bookshelves I built square they worked great. When the glue was dry I removed the squares and measured diagonally. Result? The hulls were perfectly square. Highly recommended.