In archaeological research, onion-knob fibulae are primarily classified according to the typology of Keller and Pröttel into six main types (types 1 to 6). Chronological classification is largely based on formal characteristics such as the proportions of the bow, the design of the foot (e.g., decoration with circular eyes), the shape of the eponymous knobs, and the fastening mechanism (hinge). Furthermore, inscriptions on imperial ceremonial examples made of precious metal allow for the precise dating of individual pieces.