The coin is known as an "EID MAR" because of that very inscription, on the back of the coin. The reference is to the Ides of March, or March 15, the date in 44 B.C. on which the famed Roman general and politician Caesar was murdered, by Brutus and other members of the Roman Senate. Also on the back of the coin are a pair of daggers, the Senators' weapon of choice in killing Caesar, and a liberty cap, recognized by many as a symbol of freedom. The coins are also known as Denarii of Brutus.

Brutus and Caesar were two of many powerful men in the waning days of the Roman Republic. Caesar found great success on the battlefield, subjugating Gaul and then defeating his greatest rival, Pompey, in what became a civil war after Caesar violated sacred tradition by crossing the Rubicon at the head of an army. Caesar accepted the title of Dictator in Perpetuity, and many in Rome, powerful and otherwise, feared that he wanted to become king.

One of the things that cemented the decision of Brutus, Cassius, and others to violently remove Caesar from power was what they took as hubris when he became the first living Roman to mint his own coins with his portrait on them. He did this in 44 B.C., the year of his death.

Significantly, the EID MAR coins that Brutus had minted to pay his troops during the Battle of Philippi, a triumph by Caesar's nephew Octavian (later Augustus) at the hands of Brutus and Cassius, have Brutus's portrait on the front.

Brutus had the coins minted in different metals. Of the silver ones, only about 100 are known to have survived. Even rarer are the gold EID MAR coins, of which only three are known to still exist.

Brutus had some experience in minting coins, having a decade earlier served as triumvir monetalis, a person in charge of such activities for the Republic.

The silver EID MAR coin is under auction on August 15 and is expected to sell for up to US$300,000.