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The inch (abbreviation: in or ″) is a unit of length in the (British) imperial and United States customary systems of measurement now formally equal to 1⁄36 yard but usually understood as 1⁄12 of a foot. Derived from the Roman uncia ("twelfth"), inch is also sometimes used to translate related units in other measurement systems, usually understood as deriving from the width of the human thumb.
A metric foot is a nickname for a preferred number length of 3 basic modules (3 M), or 30 centimetres (11.811 in). The 30 cm metric ruler was a similar length to the traditional imperial one-foot ruler. A metric foot is 4.8 millimetres (0.189 in) shorter than an imperial foot.