The pitch of a roof is its vertical 'rise' over its horizontal 'span'. However, most often a ratio of "pitch" (also fraction) is slang used for the (more useful) 'slope' (of rise over 'run') of just one side (half the span) of a dual pitched roof. This is the 'slope' of geometry, stairways and other construction disciplines, or the trigonometric arc tangent function of its decimal fraction. In the imperial measurement systems, "pitch" is usually expressed with the rise first and run second (in the USA, run is held to number 12). In the USA the rise is rationalized to a ratio of so many measuring units rise to each 12 measuring units of run (e.g., 3:12, 4:12, 5:12). Places that use metric measurement systems use a degree angle, or what fall there is per unit of run, and expressed as a '1 in _' slope (a '1 in 1' equals 45°). Where convenient, the least common multiple is used (e.g., a '3 in 4' slope, for a '9 in 12' or '1 in 1 1/3').