This rule represents a very important class of slide rules - those used for air navigation. While flying,  the direction that an aircraft follows over the ground is a function of its air speed and heading and the wind speed and direction. In addition to allowing a pilot to calculate what his/her resulting track over the ground is, this rule also enables other important calculations to be carried out such as the relation between indicated airspeed and true air speed. These calculators have had a longer life than most slide rules and some are still sold commercially.