This United States 20-cent definitive stamp honors Joseph Priestley, the eminent 18th-century scientist best known for the discovery of oxygen and his pioneering work in chemistry and natural philosophy. Issued as part of the prestigious Great Americans Series, the stamp celebrates individuals who made lasting contributions to science, culture, and society. The warm monochrome engraved portrait style reflects the classic definitive design tradition of the 1980s USPS issues. Highly collectible, this stamp appeals strongly to thematic collectors specializing in science, chemistry, inventions, or notable personalities.