Porphyry column with two embracing older Tetrarchs 

Rome, 293-305

The groups have always been interpreted as perfect representations of the Tetrarchic ideology. The older emperors are the Augusti, the younger ones the Caesares. The difference of age casts them as fathers and sons (which they in fact were since the Caesares were forced to marry daughters of the Augusti). The embrace between the emperors of the same status indicates concordia (former communis opinio). This greeting and acceptance between persons of the same rank (Laubscher) demonstrates further their much admired consensus. Above all the similarity of the Augusti and Caesares respectively expressed a conception, where similitudo of function and rank was more important than personalities.
Although this remains true, slight personal characterizations in the Vatican tetrarchs can be noticed in this group. The characterisations are quite explicit. In the first Tetrarchy the Caesar on the proper right (the one who puts his arm around his colleague) should be Constantius Chlorus, whose hooked nose is known through his coin portraits and his marble portraits from Rome and was inherited by his son Constantine. In the Vatican group his nose is distinctly hooked in comparison to that of his colleague and has the characteristic sharp lines surrounding the nostrils. 
This allows us to consider the differences between the faces of the two Augusti as intentional too. Diocletian’s face (on the proper right) is much more lined and haggard than Maximianus Herculeus’ (on the proper left). From the coins it is known, that Maximianus was slightly fat. He also had an unmistakable snub nose, which is unfortunately broken in the Vatican group. Galerius probably has no unmistakable characteristics in this group. These observations confirm the general scholarly opinion that the Vatican groups represent the emperors of the first tetrarchy.
The Vatican groups are characteristic works of the Egyptian workshops, which were ordered to produce porphyry sculpture for the decentralized empire of the Tetrarchs and also exported to Italy. Typical among other traits is the rope-like structure of the folds of their paludamenta, and in the case of the Vatican groups the combination of the frontal figures with feet set more or less in profile in an Egyptian way.
The emperors wear conventional muscle cuirasses, paludamenta, swords and laurel-wreaths with oval gems in their middle. A late-antique element of dress are the long sleeves of the tunicae. The strange boots are probably supposed to be calcei senatorii, but are similar to boot-forms represented in Greco-Roman Egypt. All emperors hold globes/orbs in their left hands.
The heads with identical short cropped hair and beards follow two types. One shows an older man with a deeply lined face, lacrymal sacks, and lips pressed together; the other is a younger type with less lines and lips that seem to be smiling.