As the name implies, this is a square ruler used to make sure a patchwork block is perfectly square.  Square-up rulers are typically sized on a half inch measurement scale (i.e. 4.5″, 5.5″, 6.5″, etc.) to account for the seam allowance. For example, if the block is supposed to finish out in the quilt as a 6″ block, you would use a 6.5″ square-up ruler to make sure the block is square.