The knot (kn) is a unit of speed used at sea, defined as one nautical mile per hour. The international knot (1.852 km/h or 1.15 mph)) used in the US differs slightly from the UK knot (6,080 ft/h). The name comes from the old practice of measuring speed at sea by counting the number of knots in a knotted line payed out over the stern in a given time. The speed of aircraft, and that of winds and currents, are also measured in knots.