A pin or clasp used to in relation to clothing.
The earliest fibulae date from the Iron Age. Fibula styles changed over time and show regional variations. Pins could be hinged or they might be either straight, curved, or wound springs. Fibulae could be plain, engraved, plated, enamelled, or jeweled. Designs might be simple, abstract or zoomorphic. Size varied greatly.
Broadly speaking there were three main types of fibulae: bow, plate and ring.
This is the "safety pin" shape and all of its variations.
This is the variety most like the modern "brooch".
The ring and pin configuration remained popular in the post-Roman period. The ring might be solid (annular) or it might have a gap (penannular).
Although fibulae are often illustrated in the "pin downwards" orientation, this might not have been the norm for fibulae. A zoomorphic plate brooch  ("SWYOR-BFE1C5" in the UK "Portable Antiquities Scheme") shows a seated dog. The reverse clearly shows the pin attachment (with remains of pin) at the bottom, while the clasp portion is attached behind the dog's neck.
- Roman Era in Britain by John Ward (1911) Chapter XIV: "Dress and the Toilet"