It was in part spurred on by a New York billiard table manufacturer who announced a prize of $10,000 for a substitute material. The first viable substitute was celluloid, invented by John Wesley Hyatt in 1868, but the material was volatile, sometimes exploding during manufacture, and was highly flammable. There are many sizes and styles of pool and billiard tables. Generally, tables are rectangles twice as long as they are wide. Most pool tables are known as 7-, 8-, or 9-footers, referring to the length of the table's long side. Full-size snooker and English billiard tables are long on the longest side. Pool halls tend to have tables and cater to the serious pool player. Pubs will typically use tables which are often coin-operated. Formerly, tables were common, but such tables are now considered antique collectors items; a few, usually from the late 19th century, can be found in pool halls from time to time. Ten-foot tables remain the standard size for carom billiard games. The slates on modern carom tables are usually heated to stave off moisture and provide a consistent playing surface. The length of the pool table will typically be a function of space, with many homeowners purchasing an table as a compromise. Full-size pool tables are 4.5 by (interior dimensions). High-quality tables have a made of thick slate, in three pieces to prevent warping and changes due to temperature and humidity. Smaller bar tables are most commonly made with a single piece of slate. Pocket billiards tables of all types normally have six pockets, three on each side (four corner pockets, and two side or middle pockets). All types of tables are covered with billiard cloth (often called "felt", but actually a woven wool or wool/nylon blend called baize). Cloth has been used to cover billiards tables since the 15th century. Bar or tavern tables, which get a lot of play, use "slower", more durable cloth. The cloth used in upscale pool (and snooker) halls and home billiard rooms is "faster" (i.e., provides less friction, allowing the balls to roll farther across the table ), and competition-quality pool cloth is made from 100% worsted wool. Snooker cloth traditionally has a nap (consistent fiber directionality) and balls behave differently when rolling against versus along with the nap. The cloth of the billiard table has traditionally been green, reflecting its origin (originally the grass of ancestral lawn games), and has been so colored since at least the 16th century, but it is also produced in other colors such as red and blue.