Because of the limited nature of the Latin praenomen, the cognomen developed to distinguish branches of the family from one another, and occasionally, to highlight an individual's achievement, typically in warfare. One example of this is Gnaeus Pompeius Magnus, whose cognomen Magnus was earned after his military victories under Sulla's dictatorship. The cognomen was a form of distinguishing people who made important feats, and those who already bore a cognomen were awarded another exclusive name, the agnomen. For example, Publius Cornelius Scipio received the agnomen Africanus after his victory over the Carthaginian general Hannibal at Zama, Africa (Africanus here means "of Africa" in the sense that his fame derives from Africa, rather than being born in Africa, which would have been Afer); and the same procedure occurred in the names of Quintus Caecilius Metellus Numidicus (conqueror of Numidia) and Quintus Caecilius Metellus Macedonicus.
In contrast to the honorary cognomina adopted by successful generals, most cognomina were based on a physical or personality quirk; for example, Rufus meaning "red-haired" or Scaevola meaning "left-handed". Some cognomina were hereditary (such as Caesar among a branch of the Julii, Brutus and Silanus among the Junii, or Pilius and Metellus among the Caecilii): others tended to be individual. And some names appear to have been used both as praenomen, agnomen, or non-hereditary cognomen. For instance, Vopiscus was used as both praenomen and cognomen in the Julii Caesares; likewise Nero among the early imperial Claudii, several of whom used the traditional hereditary Claudian cognomen as a praenomen.
The upper-class usually used the cognomen to refer to one another.
Today, we refer to many prominent ancient Romans by only their cognomen; for example, Cicero (from cicer "chickpea") serves as a shorthand for Marcus Tullius Cicero, and Caesar for Gaius Julius Caesar.