This is a question I've been working on reassessing for years now, and I intend to eventually make it my PhD Thesis.

What we call the Theme System today is largely heavily biased by the outdated work of George Ostrogorsky. Ostrogorsky took a look at the Novellae of Constantine VII and later emperors, and found these laws regarding "military lands" (stratiotika ktemata). He made the argument that these lands had a hereditary service requirement attached to them, and that the families working these lands supplied and supported a soldier who was called up to service for campaign.

We now know that this is not how military service worked in the empire, nor is this what the "Theme System" actually was.

The Theme System actually goes back, much, much further, with the most recent works by John Haldon ("A Context for two 'Evil Deeds': Nikephoros I and the Origins of the Themata") and Salvatore Cosentino ("Land and Military Service in the Ninth Century: A Note on Nicephorus and Charlemagne"). The Theme system originates in 7.13.7 of the Codex Theodosianus, a law issued by Valens in 375 AD, which fixes the cost of recruitment at 36 solidi and the cost of clothing and personal necessities within that figure at 6 solidi. It states that only those whose value is "quorum iugatio ita magna est" have to field a single recruit, while those who were not assessed to be worth a great amount retained the right to furnish a recruit jointly. Although Dr. Cosentino does not believe this law is the same as the one enacted by Nikephoros I in 809/810, or by Charlemagne in 808 AD, I believe it is closely related, as a law almost exactly the same is also issued by Alfred the Great in the 10th century. The law may not have been in continuous effect, but it is clear what legal source they drew on as the basis for their "Theme Systems."

So what was the Theme system? It was a law that stated that:

...poor people should be enrolled in the army and should be fitted out by the inhabitants of their commune, also paying to the Treasury 18 ½ nomismata per man plus his taxes in joint liability.

- Theophanes

However, in the Memorandum de exercitu in Gallia occidendali preparando of the Capitularia, Charlemagne identifies the various combatants in relation to the values of their property. Anyone possessing two mansi of property value are instructed to associate with those of similar value, or with someone of one mansus, so that they may both furnish a recruit together. Those possesing one mansus each are instructed to group together in threes to furnish a recruit, and those possessing a half mansus should group in six to furnish a recruit. Finally, those who do not own land, the pauperes, are each required to group in five to furnish a group, providing five solidi of account each, or 300 Carolingian denarii. The Capitulare missorum de exercito promovendo is similar, setting the limit at four mansi instead of three mansi, and instructing landowners to coordinate accordingly.

When we compare the exchange rates of coinage and social status of those mentioned in the Capitularia and in Theophanes, we see a relationship. The words pauperes and ptokhoi have a similar meaning, referring to those who do not own land sufficient to support military service, and only own a small amount of movable goods. In the Carolingian world, it was the socially weak, while in the Roman world (since there were no land-based social classes starting around this time) this referred to those below a certain value of capital assessment, probably near the subsistence line. There are 1.7gr to the Carolingian Denarius to the 2.3gr of the Roman Miliaresion, which puts 300 Denarii at equivalent to 18.5 Nomismata (222 Miliaresia). This puts the cost of furnishing a recruit in the empire (6 men - five only paying, one serving) at 37 Miliaresia per man, or about 3.08 Nomismata per man, just below the estimated subsistence line of 3.5 to 3.7 Nomismata.

This is Nikephoros' Thema, a word simply derived from the 7th century term for a large army (called exercitus in Latin). And therefore, this is the actual Theme system. But this only answers part of your question, which I'll follow up in the next post since I'm probably nearing the word limit:

(1/2)