Identifying brooch and fibula types
Spring, bow, and catch.
A fibula works like a safety pin: spring or hinge, bow, and catch-plate. The form of each is closely datable, from La Tène through Roman to early medieval, which makes brooches some of the most precisely datable metal finds there are.
Note whether the pin is sprung or hinged, how many coils the spring has, whether its chord passes inside or outside, and how the catch is made — open hook or solid plate. Each detail moves the date.
The great Roman families are the bow brooch, the flat enamelled plate brooch, and — late in the sequence — the crossbow brooch with its three knobs, a virtual badge of late Roman officialdom.
Compare against a regional type series and record the spring, bow section, and catch separately. A fibula missing its pin is still closely datable; say what survives.