Standard lengths on the wall of the Royal Observatory, Greenwich, London – 1 yard (3 feet), 2 feet, 1 foot, 6 inches (1/2-foot), and 3 inches. The separation of the inside faces of the marks is exact at an ambient temperature of 60 °F (16 °C) and a rod of the correct measure, resting on the pins, will fit snugly between them.