Nm

The Metric Number (Nm) is a common system for specifying the fineness of sewing thread based on a fixed weight. It defines the length of the thread in meters that weighs exactly one gram; for instance, Nm 40 means 40 meters of that thread weigh one gram. Because of this calculation, the system works inversely to thickness: a higher number indicates a finer thread, while a lower number indicates a thicker, heavier thread.