Dr. David Gwynn

And quite basically, Diocletian doubled the number of provinces without taking a single territory.

He, in other words, splits almost all of them in half.

So he makes the provincial structure more complex and much smaller.

At the same time, he also makes it very clear, something that Constantine will then continue, that you've got provincial governors and you've got local army commanders and they're different people.

Exactly.

So separate out civilian and military affairs.

Again, it had been happening in the third century, but very erratically.

Why is he doing this?

Step one, smaller provinces are easier to control.

So a local official is going to be unable to develop a major power base.

It's also therefore easier for a local official to administer.

smaller groups are easier to control.

You do, of course, sometimes need to then deal with provinces as a group.

So another layer gets added.

Interestingly, given that the terminology will all be taken over by the Christian church, these are the diocese.

And a Roman diocese under Diocletian is a block of provinces with one official over all the provincial governors whose title is vicarius.

Is the origins of the word diocese from Diocletian or is that too much of a stretch?

It's not from the name Diocletian, but it's from this provincial division, just as the person in command of a diocese is a vicar.

The title is actually vicarius.

Because you need that layer now.