Roman military diploma

[1] The diploma was a notarised copy of an original constitutio (decree) issued by the emperor in Rome, listing by regiment (or unit) the eligible veterans.

The constitutio, recorded on a large bronze plate, was lodged in the military archive at Rome (none such has been found; presumably they were melted down in later times).

Such corps were: the auxilia; Roman navy, the Praetorian Guard's cavalry (equites singulares Augusti); and the cohortes urbanae (the City of Rome's public-order battalions).

[2] The first known diploma dates from AD 52, under emperor Claudius (r. 41–54), who appears to have regularised the practice of granting Roman citizenship to non-citizen auxiliaries after 25 years' service (26 in the navy).

[3] In 212, the Constitutio Antoniniana, issued by the emperor Caracalla, granted Roman citizenship to all the inhabitants of the empire, thus ending the second-class peregrini status.

But diplomas for service in the navy, Praetorian Guard cavalry and the cohortes urbanae continued to be issued until the late 3rd century.

The veteran was granted Roman citizenship, which carried important legal and fiscal advantages, including exemption from the poll tax (tributum capitis) payable by all non-citizen subjects of the empire.

An exceptional constitutio of emperor Hadrian (r. 117–38) is known from 3 diplomas, which granted citizenship to the beneficiaries' parents and siblings, in addition to their children.

In addition, diplomas usually record the names of several auxiliary units which served in the same province at the same time, as they were normally issued in batches.