Mug was 18th century street slang for a face. Add the word shot and everyone pictures the same thing: a plain background, flat lighting, and a front view set beside a profile. This episode traces how that format came to be. Police in Belgium were photographing prisoners by 1843 and 1844, only a few years into the age of the daguerreotype, and the practice soon reached Sydney, Liverpool, and Birmingham. By 1853 Philadelphia police had a gallery of criminal portraits on its walls. The trouble was that these early images looked like Victorian family portraits, with hats, shadows, and wistful poses that made them nearly useless for identification.

The fix came in 1888 from French police officer Alphonse Bertillon, who stripped out every artistic choice and treated the face as measurable data. His two-view standard spread across Europe, Russia, and the United States. It worked so well that it created a new problem: the format became shorthand for criminality, and American courts now treat the photo itself as a threat to a fair trial.

• Alexander Gardner photographed the Lincoln assassination conspirators in 1865 and had them turn their heads for stark profile views, an early step toward the standard.
• The profile captures what a frontal shot cannot: the slope of the forehead, the projection of the nose and chin, and the shape of the ear, which changes little in adulthood.
• In Barnes v. United States, a federal appeals court said the double-shot picture is so familiar that jurors infer a criminal record on sight.
• Illinois requires booking photos shown in court to have placards and height charts removed and dates stripped, and lawyers may not call them mug shots.
• Research on six-pack photo arrays shows grouped pictures produce fewer false identifications than a single photo, because witnesses compare faces instead of answering yes or no.