A Milliarium (plural milliaria) was a stone that was placed alongside Roman roads. Such stones were used from about the 3rd century BC. They marked the distance between two towns, and were placed at intervals of one Roman mile. This was about 1000 milia passum, or 1.5 kilometres (0.93 mi). In the celtic provinces, leagues are often used, these are 1500 milia passum, or about 2.2 kilometres (1.4 mi).
Very often the stones condain the following information:
- The name of the person or emperor who built or repaired the road, usually with all titles.
- The distance between the starting point, and the point where the millarium was placed. Some milliaria liste the distance to a larger settlement instead.
Even if they look like modern road signs showing the distance to given places, this was probably not the first function of these stone pillars. What seems more likely is that they were used to show the power of the person who erected them; some were probably used for propaganda. Very often, the inscription of the emperor was in Latin, but the inscription of the distance was in Greek. The ordinary population could probably read the second part, but not the first. Today between 7.000 and 8.000 such stones are known.