Samuel Clemens, aka Mark Twain (1835–1910), age 15 holding metal type in a composing stick that spells out his first name. He understood that the photographic printing process reversed the contents of an image in the same way backwards moveable type was reversed in printing to give clear copy.

Portrait of Samuel Clemens aka Mark Twain as a young man holding a printer’s composing stick with letters SAM, 1850.