The knot (kn) is a unit of speed used at sea, defined as one nautical mile per hour. The international knot (1.852 kilometers per hour or 1.15 mph)) used in the US differs slightly from the UK knot (6,080 feet per hour). 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.