Later usage has sometimes regarded the terms "labarum" and "Chi-Rho" as synonyms (i.e. the labarum bearing the chi-rho symbol). Ancient sources however draw an unambiguous distinction between the two, as since the "Chi-Rho" [[w:Christogram|Christogram]] and the "Labarum" were not originally synonyms; . As already noted, originally, the labarum being a type of [[w:Vexillum|vexillum]], was a military standard used in the Classical Era of the Roman Empire, with a flag hanging from a horizontal crossbar; the Chi-Rho Christogram was only added to the flag by the Emperor [[Constantine the Great|Constantine I]] in the late Roman period.