A book in good condition should be a rectangular solid when at rest, whether upright or on its back, with the covers at right angles to the spine. If a book is out of square, usually from resting crooked on a shelf, or leans to the right or left when on its back, it is "cocked", or "shelf-cocked". If the covers bend in or flare out, usually from rapid humidity changes, a book is "bowed" (bent like a drawn bow). Thick hardbound books also tend to have their pages sag downward in the middle even if they are sitting level on a shelf.