Staff Writer (Updated: 5/12/2014):
Generally, derringer pistols were large-caliber weapons utilizing very compact frames. They were often of single-shot or even twin-shot configurations actuated by a trigger pull and a flintlock or percussion cap arrangement (the latter seen in later models). Due to their compact size, they were perfect guns for concealment in a coat pocket, or a lady's purse, and useful in extremely close-range confrontations. Deringer produced their small pistols in matching pairs typically delivered in ornate showpiece boxes (consistent with pistols of the period). Each design varied in caliber and barrel length.
Derringer pistols have proven equally popular in Hollywood Westerns where the gambler or hero usually carries one, or a pair, in a spring-loaded contraption hidden under the sleeve or simply in a leg boot though it is reasoned that their use in real life was far more conventional.
According to sources, derringers were used to a certain extent in the American Civil War despite their not being true "military-minded" weapons.