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Length, when referring to measurement in one dimension (as in the length of a line segment or a or a piece of string), has a specialized meaning. So does height when it is used in conjunction with base.
One meter is about the length of a guitar and the width of a doorway. The French Academy of Sciences first defined the meter in 1791 as one-ten-millionth the length of Earth's meridian along a quadrant. The first prototype was 0.2 millimeters short due to a miscalculation caused by the rotation of Earth.