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The hand is a non-SI unit of measurement of length standardized to 4 inches (101.6 mm). It is used to measure the height of horses in some English-speaking countries, including Australia, Canada, the Republic of Ireland, the United Kingdom and the United States.
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.