Safety pin.
All brooches, as has been said, originated from the simple pin, which itself was preceded by and probably derived from a thorn. At an early period this pin, after having been passed through the garment, was for greater security bent up, and its point caught behind the head. Later, in order that the point might be held more securely in the catch, the pin was given a complete turn, which produced the spring, as seen in the common form of our modern safety-pin. Thus constructed, the brooch, though in one piece, may be said to consist of four parts; (a) the acus or pin; (b) the spring or hinge; (c) the catch or locking apparatus, which forms the sheath of the pin; and (d) the bow or back—the framework uniting the spring with the catch.
Romano-British brooch or fibula with bilateral spring.
From this primitive safety-pin, which is the foundation form of all brooches with a catch, developed the numerous varieties and patterns of the brooch or fibula of succeeding ages. Amongst these is the Roman fibula, which instead of being made of one piece of metal, is of two pieces—the bow and the acus. The pin here works on a hinge—the result of gradually extending the coils of the spring symmetrically on each side of the pin into what is known as the double-twisted or bilateral spring, and placing a bar through the coils thus made. From the brooch hinged in this manner originated the Roman provincial fibula of the T-shaped type common in France and Britain, and later the cruciform brooch of Anglo-Saxon times. The brooch with with a hinge was exclusively used until the revival of the "safety pin" with a spring, patented as a new invention in the nineteenth century.
Brooch formed of double spiral discs of concentric wire ("Spectacle" fibula.)
In addition to the above brooches or fibulæ (group 1)—all developments of the safety-pin type—there are three other large groups of brooches: (2) the circular disc type; (3) the penannular or Celtic brooch; and (4) the ring-brooch. The first of these—the type generally worn at the present day—may be described as a flat disc fitted with a hinged pin. In cemeteries belonging to the Early Iron Age in Southern Europe circular plates have been found fitted with a pin. These plates appear[1] to have been developed by the conversion of a primitive disc of spiral concentric wire into a circular plate. From the brooch of this type sprang the circular brooch of the Roman period, often inlaid with enamel, as well as the splendid circular brooches of Anglo-Saxon times, and all other disc-shaped brooches. In all early periods, and even in Roman times, the bow or safety-pin type of brooch was commoner than the disc and also more practical, as it offered room for the gathered folds of the garment. In modern times the disc-shaped brooch fitted with a hinged or sometimes with a spring pin has been principally used.