The knot (/nɒt/) is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h (approximately 1.151 mph or 0.514 m/s).

The chip log was cast over the stern of the moving vessel and the line allowed to pay out.

[9] The knot count would be reported and used in the sailing master's dead reckoning and navigation.

Although the unit knot does not fit within the SI system, its retention for nautical and aviation use is important because the length of a nautical mile, upon which the knot is based, is closely related to the longitude/latitude geographic coordinate system.

As a result, nautical miles and knots are convenient units to use when navigating an aircraft or ship.

On a standard nautical chart using Mercator projection, the horizontal (East–West) scale varies with latitude.

Since the length of a nautical mile, for practical purposes, is equivalent to about a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart.

Recent British Admiralty charts have a latitude scale down the middle to make this even easier.

In 1969, these standards were progressively amended to specify that distances were to be in nautical miles, and speeds in knots.