The only essential equipment in a basketball game is the basketball and the court: a flat, rectangular surface with baskets at opposite ends. Competitive levels require the use of more equipment such as clocks, score sheets, scoreboard(s), alternating possession arrows, and whistle-operated stop-clock systems. A regulation basketball court in international games is 28 metres long and 15 metres wide. Most courts have wood flooring, usually constructed from maple planks running in the same direction as the longer court dimension. The basket is a steel rim 18 inches in diameter with an attached net affixed to a backboard that measures 6 feet by 3.5 feet, and one basket is at each end of the court. The size of the basketball is also regulated.