the resin to deform and cause an uneven playing surface; it can also be difficult to move once joined. Tables for the home market usually use slate beds as well, but the slate is often thinner, down to about . The early table beds were made of cloth-covered wooden boards. Today, inexpensive but not very rigid or durable materials used for the beds of low-end tables (e.g. for children's recreation rooms) still include wood, especially medium-density fibreboard and plywood, as well as plastics and other synthetic materials under various trade names. Cloth Billiard cloth (sometimes erroneously called felt) is a specific type of cloth that covers the top of the table's "playing area". Both the rails and slate beds are covered with 21–24-ounce billiard cloth (although some less expensive 19-ounce cloths are available) which is most often green in colour (representing the grass of the original lawn games that billiards evolved from), and consists of either a woven wool or wool-nylon blend called baize. Most bar tables, which get much use, use the slower, thicker blended felt because it is cheaper. This type of cloth is called a woollen cloth. By contrast, high-quality pool cloth is usually made of a napless weave such as worsted wool, which gives a much faster roll to the balls. This "speed" of the cloth affects the amounts of and of the balls, among other aspects of game finesse. Snooker cloth traditionally has a directional nap, upon which the balls behave differently when rolling against vs. running with the direction of the nap. Markings , also known as (for their traditional shape), are inlaid at precise, evenly spaced positions along the rails of some tables (not usually on snooker tables) to aid in the aiming of bank or kick shots. There are seven along each long rail (with the side pocket interfering with where the seventh one would go, on pocket billiard tables) and three along each short rail, with each of the four corners counting as another in the mathematical systems that the diamonds are used to calculate. These sights divide the playing surface into equal squares. Books, even entire series of books, have been written on geometric and algebraic systems of aiming using the diamonds. Spots are often used to mark the and on the cloth. Other markings may be a line drawn across the (or across the with , in British-style pool).