The most common way to display bound materials is closed and lying horizontally. If a volume is shown open, the object should be open only as much as its binding allows. Common practice is to open volumes at an angle no greater than 135°. There are some types of equipment that help support volumes as they displayed openly: blocks or wedges, which hold a book cover to reduce stain at the book hinge; cradles, which support bound volumes as they lay open without stress to the binding structure; and polyester film strips, which help to secure open leaves. Textblock supports are best used in conjunction with book cradles where the textblock is greater than 1/2 inch, or where the textblock noticeably sags. Regardless of its method of support, however, it is worth noting that any book that is kept open for long periods can cause damage. One should turn an exhibited book's pages every few days in order to protect pages from overexposure to light and spread any strain on the binding structure.