The title's date of establishment is unknown, but it first appears in inscriptions of the Late Republic, from c. 80 BC onwards. Previously, the official name of the Roman state, as evidenced on coins, was simply ROMA. The abbreviation last appears on coins of Constantine I the Great (ruled AD 312-37),the first Christian Roman Emperor.
The two legal entities mentioned, Senātus and the Populus Rōmānus, are sovereign when combined. However, where populus is sovereign alone, Senātus is not. Under the Roman Monarchy neither entity was sovereign. The phrase, therefore, can be dated to no earlier than the foundation of the Republic.
This signature continued in use under the Roman Empire. The emperors were considered the representatives of the people even though the senātūs consulta, or decrees of the Senate, were made at the pleasure of the emperor.
Populus Rōmānus in Roman literature is a phrase meaning the government of the People. When the Romans named governments of other countries they used populus in the singular or plural, such as populī Prīscōrum Latīnōrum, "the governments of the Old Latins". Rōmānus is the established adjective used to distinguish the Romans, as in cīvis Rōmānus, "Roman citizen". The locative, Rōmae, "at Rome", was never used for that purpose.
The Roman people appear very often in law and history in such phrases as dignitās, maiestās, auctoritās, lībertās populī Rōmānī, the "dignity, majesty, authority, freedom of the Roman people". They were a populus līber, "a free people". There was an exercitus, imperium, iudicia, honorēs, consulēs, voluntās of this same populus: "the army, rule, judgments, offices, consuls and will of the Roman people". They appear in early Latin as Popolus and Poplus, so the habit of thinking of themselves as free and sovereign was quite ingrained.