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The millimetre (International spelling as used by the International Bureau of Weights and Measures; SI unit symbol mm) or millimeter (American spelling) is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length. Therefore there are thousand millimetres in a metre. There are ten millimetres in a centimetre.
An ell (from Proto-Germanic *alinō, cognate with Latin ulna) is a unit of measurement, originally a cubit, i.e., approximating the length of a man's arm from the elbow ("elbow" means the bend or bow of the ell or arm) to the tip of the middle finger, or about 18 inches (457 mm); in later usage, any of several longer units.