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 the "Chi-Rho" [[w:Christogram|Christogram]] and the "Labarum" were not originally synonyms; 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.
In addition, the "Chi-Rho" Christogram was not always used is connection with the imperial labarum, but its use by Christians naturally evolved into a variety of formats, including on coins and medallions (minted during Constantine's reign and by subsequent rulers), on Christian sarcophagi and frescoes from about 350 AD, becoming part of the official imperial insignia after Constantine, eventually appearing on public buildings and churches as well.
A later Byzantine manuscript indicates that a jewelled labarum standard believed to have been that of [[Constantine the Great|Constantine]] was preserved for centuries, as an object of great veneration, in the imperial treasury at [[Church of Constantinople|Constantinople]].<ref>Lieu and Montserrat p. 118. From a Byzantine life of Constantine (BHG 364) written in the mid to late ninth century.</ref> The labarum, with minor variations in its form, was widely used by the Christian Roman emperors who followed Constantine I.