As I’ve commented before, films about ancient Rome always get the names wrong (or nearly always). Perhaps because it’s a TV show based on a novel drawn heavily from Roman sources, I, Claudius doesn’t butcher Roman naming conventions by just making up whatever nonsensical Roman-sounding names the screenwriter wants. So kudos to Robert Graves and Jack Pulman! But that doesn’t mean the show gets the names exactly right.
Several of the characters are not referred to properly. To refresh your memory, upper-class Roman male names have three components: the praenomen, the nomen, and the cognomen. The praenomen was a man’s private, personal name, like the Western ‘first’ name. It was used by his family and his closest friends in private, but was not generally used publicly. His nomen was his clan’s name, essentially the Western ‘last’ name, and was the normal way to refer to a man in public. The cognomen was used to distinguish that specific man from the various other men who might have his praenomen and nomen. It was often a nickname based on a significant feature of his body (Ahenobarbus means ‘bronze-beard’, for example), but it could also be a reference to a personality trait or something else. A man’s cognomen sometimes became so strongly associated with him that it became hereditary, so that Ahenobarbus’ descendants would have that cognomen without having bronze beards. The Romans also sometimes granted men an agnomen, a name that indicated a great accomplishment; Scipio was granted the agnomen Africanus after defeated Carthage in the Second Punic War. Unlike cognomens, agnomens rarely became hereditary.
Claudius was not the emperor’s praenomen; it was his nomen. But throughout the series, his family refers to him as Claudius. This would be like everyone in my family called me ‘Mr. Larsen’ throughout my life. His nomen was Tiberius, and that’s what his family called him. For example, Suetonius quotes several letters written between Augustus and Livia about the boy, and they consistently refer to him as Tiberius. However, it’s easy to see why the show does this. There’s already a Tiberius who’s an important character in the show, and Claudius is historically referred to by his nomen rather than his praenomen, so it would be confusing to viewers.
Several other characters are also referred to using names that it’s unlikely their families would have employed. Postumus was a traditional cognomen for men who were born after their father’s death. Marcus Agrippa Postumus would have been called Marcus, but the show already has a Marcus Agrippa and a Herod Agrippa, so clearly the show decided to call him by his cognomen so viewers wouldn’t be confused.

John Castle as Postumus
The character the show calls Germanicus (played by David Robb) was probably born Nero Claudius Drusus, but all three of those names are used by other characters. Scholars aren’t clear what his actual birth name was, because the sources consistently use his agnomen Germanicus. His father referred to as both Nero Claudius Drusus and Drusus Claudius Nero and in the show simply as Drusus (Ian Ogilvy) received that agnomen for his conquests in the Rhineland. The Julio-Claudians had a tendency to treat agnomens as hereditary, and so Drusus’ son wound up with it as an agnomen. The sources pretty consistently call him Germanicus so that’s how the show names him, even though that probably isn’t how his family called him.
Something similar happens with Claudius’ son Tiberius Claudius Caesar. When Claudius’ troops conquered Britain, he was accorded the agnomen Britannicus. He turned it down for himself and awarded it to his son instead. The show calls this boy Britannicus, whereas his family probably called him Tiberius, although given that that was his father’s praenomen as well, they might have called him Britannicus simply to distinguish him from his dad.
In a rather different vein, the emperor Caligula’s full name was Gaius Julius Caesar Augustus Germanicus. Caligula was actually an agnomen he picked up as a child. As a small child, he lived in his father Germanicus’ military camp, and Germanicus seems to have dressed him like a Roman soldier, including having child-sized military sandals, caligae, made for him. Caligula is a diminutive form of caliga, meaning literally ‘Little Boots’. It was a nickname the soldiers gave him, and as an adult, he despised it. In the series, everyone refers to him as Caligula. When he was a child, that’s plausible, but as an adult, it’s unlikely people called him that to his face, especially after he became emperor. But Caligula is how everyone thinks of him today so the show uses that. (I tend to think of him as Gaius Caligula.)
Probably the most egregious example of this happens with Castor (Kevin McNally), the son of Tiberius. His name was Drusus Julius Caesar. Castor was an agnomen he picked up in his early 20s when he got into an argument with someone and punched the name. The gods Castor and Pollux were associated with boxing, so Castor was a joking reference to that incident. But in the series, he’s called Castor even when he’s a young boy, long before he acquired the nickname. The obvious reason for this is that the show already has a Drusus.

Kevin McNally’s Castor
The show makes a pretty reasonable set of choices here. Roman naming practices were getting pretty confusing by the early Principate; the Julio-Claudians repeatedly changed their names and the use of agnomens got somewhat out of hand because they were trying to emphasize how militarily successful they were. They also wanted to shoe-horn in the fact that they were legally descended from Julius Caesar and Augustus, because they wanted to draw on the popularity of those two. And the practice of emperors adopting their successors caused name changes as well. If the show had tried to be historically accurate with names, the viewers would have been bewildered. The fact that several characters had the same praenomen would also have led to confusion. And Caligula and Claudius are known by those names and not their praenomens. So I think the show has good justification for fiddling around with the names of its characters.
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