The yard is the traditional length unit of linear measure. It is traditionally equivalent to 0.9144 meters, though it has been varied between 9 and 12 feet in recent times. The yard was first conceptualized by fifth-century BC Egyptians and was standardized as 9 feet in 1795 by the French military engineer General Hoche. During the American Revolutionary War, the Continental Army was unimpressed with various proposals for a new foot measuring system and instead adopted the British yard as their standard. The use of the yard spread rapidly throughout the world after that due to its international compatibility. Today, most countries use either the metric system or a variant of it, and there are few remaining areas where yards are used; Africa mostly uses meters, India uses centimeters, parts of Asia use traditional units like li and fen.

The kilometer (from the Greek word for “thousand”) is a unit of length, equal to 1000 meters. It was first proposed in 1875 by the French scientist Georges Le Verneur, and was first used in 1880. The SI base unit of measurement is the meter, but since 2006, the kilometer has been recognized as an official unit of measure by the General Conference on Weights and Measures (CGPM), alongside the meter, kilogram and second. The kilometer is not officially defined, but it is based on the distance that light travels in a vacuum in 1/299.792 of a second.