The obverse or front legends of the 3rd and 4th century coins are the easiest to read.
They take the typical form:
IMPMIVLPHILIPPVSAVG = IMP M IVL PHILIPPVS AVG (3rd century)
GALLIENVSAVG = GALLIENVS AVG (3rd century)
IMPPOBVSPFAVG = IMP PROBVS PF AVG (3rd century)
DNCONSTANTIVSPFAVG = DN CONSTANTIVS PF AVG (early 4th century on)
These are from coins of Philip I, Gallienus, Probus and Constantius II. It is easy to find the name of the emperor, which is the first step in attributing and dating a coin; it is usually stuck between the abbreviations IMP (Imperator) or DN (Dominus Noster = our lord) at the beginning of the legend and AVG (Augustus) or PF AVG (PF = Pius Felix) at the end. One can also find a few additional letters after the IMP. These additional letters after IMP stand for the other names of the emperor, in the case of Philip I above, the M IVL is for Marcus Julius. Coins up to the early 3rd century sometimes have a title such as BRIT, GERM or PART, often just before or after the AVG. Finally, sometimes the IMP, DN or PF were not used, which makes it even easier. Note that names willoften end with ’VS’. If you can spot these, you can find the name!
From the 1st to early 3rd century, the obverse legends were also frequently crowded by additional titles or offices that the emperor held. These included Consul (COS), Tribune (TRP), Pater Patriae (PP) or Pontifex Maximvs (PM). Also, titles for victories in military campaigns, such as BRIT(annicus), GERM(anicus) or PART(icus) can be seen, along with an IMP at the end of the legend which indicates a more general victory and is also often followed by a number. The numbers indicate the number of years for which the office was held, which makes exact dating of the coins possible. You might see these titles placed on the reverse of the coin, a common practice in the 3rd century, so you might find IMP M IVL PHILIPPVS AVG on the obverse, and PM TR P II COS PP on the reverse.
The below obverse legends belong to Domitian, Hadrian and Septimius Severus:
IMPCDOMITAVGGERMPMTRPXII = IMP C DOMIT AVG GERM PM TRP XII
HADRIANVSAVGCOSIII PP = HADRIANVS AVG COS III PP
LSEPTSEVPARTAVGIMPV = L SEPT SEV PART AVG IMP V
The emperor’s name is to be found among the obverse legend inscriptions. However, just as we have more than one name, so did the Romans, and it’s nice to read them from your coins. The Tria Nomina, or the Roman naming system, is fairly easy to understand.
The Latin word for name is ’nomen’, and a name consisted of three parts, the so called praenomen (prae is Latin for pre, hence pre-name), nomen (name) and cognomen (cog is Latin for additional, hence additional-name). This is just like our first, middle (mother’s maiden often) and last (family) name. Emperors are known in history by their last names, or family names in most cases, with few exceptions such as Nero or Titus for example. Since the first name is almost always abbreviated, often to just one letter, that makes it easier if one is only concerned with the emperor’s historical name.
The coins from the second half of the 4th century are the easiest in that respect, as they never included any additional names, and during first half of the fourth century a few included other names, and then it was only one of these: FL, GAL, IVL, LIC. During the first to third century however the other names will often be included in the legend.
One might be interested in knowing what the other names were, or be certain that the legend translation is accurate. In such a case it is necessary to know what all the other letters mean. 3rd century coins are full of abbreviations of the first and middle names, with the first abbreviated to one letter in many cases, such as MCL (Marcus Claudius) or CVAL (Caius Valerius). 1st and 2nd century coins will often partially spell out the first name; SER GALBA (Galba), or just use the first name as did Tiberius; TI CAESAR. While confusing at first, it does get easier comes with time.
If, like many collectors, you choose to start with the easier 4th century and then move back in time, enough knowledge will be accumulated to deal with the challenges as you go along. Below is a table of the most common and not so common abbreviations for the praenomina. The word Imperator is not properly a first name, but a title, however, it was used in place or in addition to the first name, hence many coins start with it. The same thing goes for Caesar, which was an adopted name by emperors and heirs to the throne.
Sometime you may not be certain if some letters stand for a name or a title. For example, lets look at IMPCMAVR NVMERIANVS or IMPCMQ TRAIANVS DECIVS. Does the C here stand for Caesar, or Caius ? (by the way, this name is often spelled as Gaius). Well, in the first case it must stand for Caesar because it comes from a coin of Numerian, whose name started with Marcus, and so it could not be Gaius. But, in the second case of Trajan Decius, his name did start with Gaius, so the C stands for that, right? No! Confusing, yes… Here is the point; if three names in total are listed, the fourth letter (1st in the list of names) is always for Caesar. As a good rule, if the C comes before an M, than it will be for Caesar (first name often being Marcus; Trajan Dacius is
a rare example of Caius Messius). Another important point; if you ever see two Cs, as in IMPCCVAL, then the first C is also for Caesar.
Most of the names (the second use of M is only needed for Trajan Decius), and be able to read the entire obverse inscription. If you want to be a 100% sure, then you need to look up a list of emperors with their full family names. See for example the List of Roman Emperors, or books by Van Meter or Klawans.
Finally, starting around the middle of the 4th century it became common practice to put a break in the middle of the obverse legend, hence splitting the emperor’s name above his head. This occurred earlier too, but at random. For some mysterious reasons, this was to signify that the emperor in question was the senior emperor if more than one were in power. This helps a bit in dating the coins, as for example if two emperors were alive at the same time, the senior one could have the break, but when he died the other one would start splitting his legend, narrowing down the date of issue.