Tennis balls begin to lose their bounce as soon as the tennis ball can is opened and can be tested to determine their bounce. A ball is tested for bounce by dropping it from a height of 100 inches (2.54 m) onto concrete; a bounce between 53 and 58 inches (1.3462 – 1.4732 m) is acceptable (if taking place at sea-level and 20°C / 68°F; high-altitude balls have different characteristics when tested at sea-level). Modern regulation tennis balls are kept under pressure (approximately two atmospheres) until initially used.