This is a 1/2" (12mm) solid cast brass D ring which has been plated with nickel for a silver appearance.
12mm (1/2") is the internal measurement along the straight side of the D Ring.
These Ds have a decent depth to them, measuring 12.5mm internally from the straight bar to the centre of the curve.
Cast metal Dees are far stronger and more reliable than wire formed ones, and this is especially important when making or repairing horse tack or dog collars!
