A single copy of a map. For example, if 1000 copies of a map are printed, there will be, at that time, 1000 impressions. Occasionally, it is possible to distinguish between early and late impressions of copper engravings. Copper is soft, and tends to wear. Therefore early impressions tend to be darker, and sometimes faint lettering guidelines used by the engraver are visible on impressions made early in the plate’s life.