In the military, a "click" is a term used to denote one kilometer or 1,000 meters, 0.6214 miles or 3,280.84 feet. The term is used to facilitate communication between military members and allied forces, as the US military uses the metric system almost exclusively. The term "click" may also refer to adjusting a weapons sights in the military, as one "click" on most weapons equals one minute of arc, or one inch of distance at one hundred yards. The origin of the term "klick" is uncertain, but some theories suggest that it may have been invented during World War I or the Vietnam War.