Think of them as the wallet photos of the past — portrait miniatures were an evolution of the broader miniature illustration style used to illuminate manuscripts. Around the 1520s, in the French and English courts, tiny portraits began to replace portrait medallions in brass or silver as the business cards of the day. Where medallions could be dimensional, portrait miniatures could be colored to more evocatively depict the sitter. These tiny portraits range from just over a centimeter in height to the size of the palm of your hand, and were often encased in finely embellished lockets. For more information about portrait miniatures, check out the Victoria and Albert Museum’s history of the subject below.
Reed Enger, "Portrait Miniatures," in Obelisk Art History, Published August 13, 2018; last modified August 13, 2018, http://18.104.22.168/subjects/portrait-miniatures/.