A nautical mile or sea mile is a unit of length. It is accepted for use with the International System of Units (SI), but it is not an SI unit. The nautical mile is used around the world for maritime and aviation purposes. It is commonly used in international law and treaties, especially regarding the limits of territorial waters. It developed from the geographical mile. Since the nautical mile is roughly equal to one minute of angle at the equator, the length of the equator is roughly 21,600 nautical miles (40 000 km).