Diploma (now also called Constitutiones) is a modern word for the Roman discharge certificates that were given to soldiers of the fleets, auxilia and praetorian units. The diploma describes what grants or pivileges were given to the discharged soldier, what unit he was from, and for what service it was given. Diplomas are therefore an excellent source for the location of units, length of service and of course legal status of soldiers and veterans.

Example of (part of) a diploma, on display in the Regionalmuseum, Xanten (Germany).
 

- Form: diplomas consist of two bronze plates (16x14 cm is a good mean value; there are (few) bigger as well as smaller examples), attached with wiring like two pages of a book, engraved on both exterior and interior (though not always in the interior, but that's an exception). Interior and exterior were supposed to contain the same text, with the interior part - usually bound shut - as a check for the exterior.
- The text is always a variety on: "The Emperor so and so [titles follow] grants to the soldiers of the following units [unit names] in the province [name follows] under the command of so and so the following rights: [rights follow, usually citizenship to the soldier, his 'wife' (actually, they weren't allowed to marry until after discharge until the reign of Septimius Severus) and children, but there are differences in earlier and later diplomas]. Date by consuls, place where the original decree was affixed on a temple in the forum. Granted to soldier so and so specifically, from there and there, etc. Witnesses.
- The tablets were not supposed to be broken up for any sons. Besides, they had their citizenship with that same grant (depending on when exactly they were born, but that too varies through time).
- Something like 800-1000 diplomas (or fragments thereof) have been found now, however only about half has been published. That number means they are a very important source for the Roman army, especially their lists of units have been diligently used to discover and describe troop movements throughout the empire. New bits and pieces turn up yearly, especially from the Balkans, among them a good number forgeries.
- The earliest known diploma is from the reign of Claudius and dates to 52 AD, the last one dates to January 7, 306 (RMD 78). When Caracalla gave all inhabitants of the empire their citizenship in 212, the main attraction of the diploma system was gone.

References 

- Eck, W. and Wolff, H., eds., Heer und Integrationspolitik: die Römische Militärdiplome als historischer Quelle (Cologne, 1986)
- Roxan, M., Roman Military Diplomas 1954-1977 (London 1978)
- Roxan, M., Roman Military Diplomas II 1978-1984 (London 1985)
- Roxan, M., Roman Military Diplomas III 1985-1993 (London 1994)
- Roxan, M. and P.Holder, Roman MIlitary Diplomas IV (London 2003)
- Pferdehirt, B., Die Rolle des Militärs für den sozialen Aufstieg in der römischen Kaiserzeit Monographien des Römisch-Germanischen Zentralmuseums Mainz 49 (Mainz/Bonn 2002)
- Lambert, N. and J. Scheuerbrandt, Das Militärdiplom. Quelle zur römischen Armee und zum Urkundenwesen (Stuttgart 2002)
New diplomas are regularly published in the Zeitschrift für Papyrologie und Epigraphik

An online collection of Roman diploma's can be found at RomanCoins.