Why the radiate bust
On many of our coins the emperor wears a crown of rays rather than a laurel wreath. This is not a matter of taste: it is a statement of value.
A monetary convention
The radiate crown evokes Sol, the sun god. From the first century onwards it served to distinguish a coin worth double another of the same metal: the radiate dupondius against the laureate as, for example.
When Caracalla created a silver coin worth two denarii in 215, he took up the same convention. The antoninianus would be radiate, the denarius laureate. The ancient user understood the value at a glance, without having to weigh anything.
Empresses’ busts
For the women of the imperial family, the equivalent of the radiate crown is a bust set on a crescent moon. An empress on a crescent therefore signals an antoninianus, not a denarius.
What else the bust tells you
Beyond the crown, the bust varies: draped, cuirassed, draped and cuirassed, seen from the front or from three-quarters behind, sometimes holding a spear and shield. Under Probus these variations become so numerous that they form a collecting field in their own right.
These differences are not decorative. They distinguish issues, sometimes officinae, and it is by them that catalogues separate coins otherwise identical.