There were a lot of changes to the Roman Empire during Diocletian's reign (284-305). The provinces and army were reorganized, there was a new governmental system where there were two senior emperors called Augusti and two junior emperors called Caesars, and most importantly for this post, a massive overhaul of Roman coinage. When he came to power, the coinage really consisted of heavily debased but theoretically silver radiate coins, gold coins whose weight was somewhat variable, and the local coinage of Egypt. And particularly after Aurelian unpinned the value of the gold aureus from the other coins, there was high inflation. Diocletian introduced a new silver coin that was basically an old fashioned denarius on the Neronian standard from 64 AD, and for lesser coinage, there was a small coin depicting the emperor with a laurel wreath, a new radiate coin that had no silver at all, and the big guys on the top row of those photos above, who had a small amount of silver and originally a thin silver wash that doesn't survive on most examples. Our sources refer to this big coin as the "nummus," which is just a word for coin.
It can be hard to tell how much a given ancient coin was worth if you don't have a clear literary source telling you, as it was uncommon to clearly label values for most of the Roman imperial period. But some of these coins had "XXI" on the bottom, and as I discussed in a post about another kind of coin, one possible reading of that is that this coin was worth 20 sestertii or 5 denarii. That combined with analysis of surviving prices in Egypt led to s scholarly consensus by the mid 20th century that these were worth 5 denarii, but the value assigned to them increased under Diocletian's successors, particularly Constantine I.
But artifacts don't give a flying fuck about how good and well reasoned your scholarship is, and in 1970, archaeologists found remnants of a monetary edict from September 301, well within Diocletian's reign, that states that the value of the nummus was being doubled to 25. Yes, that does mean it probably had the rather awkward value of 12 1/2 denarii at some point.1 And yes, trying to solve inflation by declaring that everyone suddenly has twice as much money as before is the most galaxy brained economic decision I've ever heard. His price control edict wasn't effective or well thought out, either.
The top row are fairly typical examples of these nummi from Diocletian's reign. The first has the most common reverse type, depicting "the Genius of the Roman People," with a "Genius" being kind of like a male divine guardian spirit (a female one would be called a Juno). The second is basically "Juno, the Guardian of Money," and it's usually thought these started being struck after the price control and/or revaluation of the currency. The final is an unfortunately double struck personification of Carthage, the city this particular coin was minted in. I'm not sure why Carthage got a whole design around the mint location when everyone else just got a mint mark, but I appreciate that they added variety on a coinage series that otherwise is mostly small variations on two reverse types.
In 305, Diocletian did something completely unprecedented for an emperor: he retired. And he forced the other senior emperor, Maximian, to retire with him. And as Diocletian intended with his new four emperor system, the Caesars were promoted to Augusti, and two new Caesars were appointed. There was just one problem: Maximian and the new Augustus Constantius I each had a son, neither of which was promoted to Caesar. And those sons were pissed. One of those sons, Maxentius declared himself emperor in Rome and started striking his own coins, a broken example of which is the first in the bottom row, and the other did the same in Britain. And that other one was named Constantine, so you probably have an idea of how the following civil wars turned out.
Even without the break, that Maxentius coin is a bit thinner and lighter than those earlier coins. The economic problems in the empire didn't cease, and all parts of it shrank their coins and reduced the silver content to pay for their armies, and at least Constantine increased the value of the coins he was striking for the same reason. Eventually, Constantine was in control of the West, while a guy named Licinius was in control in the East. And Licinius was apparently as galaxy brained as Diocletian, because he decided the best way to bring in more coins from taxation was to cut the value of all coins in half. The second coin on the bottom row is from one of his mints, and it depicts his son, Licinius II. On the reverse, it has the marking XII^ on the field, which was how they wrote 12 1/2, the "restored" value. I remember reading about a surviving letter where someone was warning a friend to spend all his money as fast as possible before word of that edict otherwise reached his city, but I'm having trouble finding that right now. If you happen to know what source that would be, let me know in the comments, and I'll update this post with it.
As far as we can tell, Constantine's reduced, valued up version of the nummus continued into the reigns of his sons, with an even smaller example from early in the reign of his son Constantius II (reigned 337-361) in the bottom row of my pictures, third from the left. But there was a further coinage reform under the joint reign of Constantius II and Constans I, one we don't have much info about, but this is probably the end of Diocletian's nummus. However, you'll notice that there's still one coin left in my picture on the far right. Constantius II was succeeded by Julian II (361-363), a man raised as a Christian but converted to paganism, who tried and failed to revive the falling fortunes of the old religion. One thing he did was to issue coins of the same size and weight as Diocletian's. While it's unclear if taking the style of coins from a pagan emperor who was famously a persecutor of Christians was itself a statement or just a coincidence of economic intention, the imagery on the reverse of these coins is probably pagan. It's a bull standing under stars, and while it's usually labelled as the Egyptian deity Apis, I've also seen it suggested that it's a more generic image of an animal that would be recognized as intended for pagan sacrifices. The inscription surrounding means "security of the state," something he probably believed was helped by the revival of pagan practice. These new large coins never really caught on, and you see them rarely after Julian's reign, and never again with pagan imagery.
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This is particularly strange since we're pretty sure the smallest unit of currency at the time was worth 1 denarius. Fuck if I know what you would get as change if you spent one of these trying to buy something priced at 10 denarii or something. Though I did see an interesting suggestion on a coin message board once that it might have been treated as 12 when used for small amounts, but that a group of 8 of them together were treated as 100, and he gave an example from I think the Netherlands of something like that happening once, but this isn't something I've seen in academic sources.