Reading an imperial legend
The obverse legend is not an ornament: it is the emperor’s identity papers at the moment of striking. Read properly, it often dates the coin to within a year or two.
The common abbreviations
IMP, for imperator, opens almost every titulature. AVG designates the Augustus, the reigning emperor; NOB CAES, or simply C, the Caesar, that is, the designated heir. P F, pius felix, dutiful and fortunate, is a formula of rule that became all but automatic in the third century.
P P stands for pater patriae, father of the fatherland. Victory titles are added after: GERM for Germanicus, PART MAX for Parthicus Maximus, BRIT for Britannicus. They point to a campaign, and therefore to a date.
The dating elements
Two mentions allow precise dating. TR P, tribunicia potestas, the tribunician power, was renewed each year: TR P XVIII means the eighteenth year of that power, and therefore an identifiable calendar year. COS, the consulship, changes less often but accumulates: COS IIII designates the fourth consulship.
Combining the two often lands you on a single year. This is how the close datings you read on our records are established.
Reading what is missing
Legends are frequently incomplete: a short flan, an off-centre strike, wear. Scholarly catalogues then restore the missing letters in square brackets. A partly legible legend is often enough for attribution, provided the reverse and the style of the portrait agree.