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Brief Commentary:

The date is the easiest part: an emperor's inscription is always exactly datable from the years of his tribunician power, which ran sequentially without interruption. Actually, I have a little confession to make; and since it will be of interest to the beginning epigraphist, here it is.

Even on the spot, I found the text hard to read, and in my little book wrote down

It was only when I got back to my books in Chicago (in this case, Sandys' Latin Epigraphy) that I could read TRIB · POT · XII for sure: you see, Trajan's 7th tribunician power ran from 10 Dec 102 to 9 Dec 103 — but that couldn't be it, since he was not proclaimed imperator for the 6th time until A.D. 106. So XII it was: 10 Dec 107 to 9 Dec 108.

The place where this milestone was originally set up is harder. First, I have no information on the actual find; it's quite possible there is none, and this stone has been lying around Orvieto for several centuries, all provenance obliterated.

Then, there's the matter of the places mentioned in the inscription:

• Clusium is easy: that's modern Chiusi, although where the borders of its territory were is a bit harder.

• Volsinii is less easy. No one is completely certain what place is meant by this ancient name, the two prime candidates being Bolsena the modern name of which obviously derives from the ancient toponym, and Orvieto, which has an abundance of Etruscan remains. See Pliny, NH 2.53.139‑140 for a catastrophe which annihilated the city.

The usual solution is that Volsinii (Novi) was Bolsena and Volsinii ("Veteres") was Orvieto: 41 and 35 km in a straight line, respectively, from the center of modern Chiusi.

Assuming a 10% rate of curvature for the road, reasonable since it traversed sparsely populated and relatively easy terrain, the maximum length of the road was either 45 (if it ran from Bolsena) or 38.5 km (if it ran from Orvieto). The only certainty is that the 17th milestone was 25.5 km by road from Volsinii.

The map drawn here, with the main modern roads, shows a circular arc centered on Orvieto, on which all points are 25.5 km from that town. The milestone must have been somewhere within this circle. It must also have been inside a 13‑km circle centered on Clusium; therefore, if it measured a distance from Orvieto, our milestone must once have stood in the shaded area O belonging to both circles.

Similarly, if it measured a distance from Bolsena, it must have been located in the yellow-shaded area B.

Since the milestone has been in Orvieto for many years, it is slightly more probable that it came from the closer of the two areas on the map; and therefore, that Volsinii was Bolsena, and not Orvieto.