A material standard used to compare spatial separations. In special relativity, a stationary equilibrium rod defines a proper length; measurements in another inertial frame require simultaneous endpoints in that frame. Geometry alone does not establish that arbitrary matter forms a stable length standard.
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A measuring rod, often referred to as a measuring stick or rod, is a straight, rigid tool used to measure lengths or distances. Traditionally made from wood, metal, or fiberglass, measuring rods provide a tactile method for gauging measurements in various applications, such as construction, surveying, and woodworking.