[[Image:Labarum.jpg|right|thumb|''Chi-Rho,'' the first two letters of [[Jesus Christ]] in Greek, which [[Constantine the Great|Constantine]] saw in a vision along with the words ''"in this sign you will conquer".'']]

The Labarum of [[Constantine the Great]] was a [[w:Vexillum|vexillum]]<ref group="note">The [[w:Vexillum|vexillum]] (plural vexilla) was a military standard (flag, banner) used in the Classical Era of the Roman Empire. On the vexillum the cloth was draped from a horizontal crossbar suspended from the staff; this is unlike most modern flags in which the 'hoist' of the cloth is attached directly to the vertical staff. The bearer of a vexillum was known as a ''vexillarius''. The vexillum was a treasured symbol of the military unit that it represented and it was closely defended in combat.</ref> that displayed the "[[w:Chi Rho|Chi-Rho]]" [[w:Christogram|Christogram]], formed from the first two Greek letters of the word "[[Jesus Christ|Christ]]" (Greek: '''ΧΡ'''ΙΣΤΟΣ, or '''Χρ'''ιστός) — [[w:Chi (letter)|Chi]] (χ) and [[w:Rho (letter)|Rho]] (ρ). Fashioned after legionary standards, it substituted the from of a [[cross]] for the old pagan symbols, surmounted by a jewelled wreath containing the monogram of Christ, intersecting '''Chi''' (χ) and '''Rho''' (ρ), on which hung a purple banner beset with gold trim and embroidery, inscribed with ''' ''"Εν Τουτω Νικα"'' (''in hoc signo vinces'') — "In this sign, conquer." '''

[[w:Lactantius|Lactantius]] states<ref>Lactantius, ''[http://orderofcenturions.org/documents/lactantius.html On the Deaths of the Persecutors],'' chapter 44.5.</ref> that, in the night before the battle, Constantine was commanded in a dream to "delineate the heavenly sign on the shields of his soldiers". He obeyed and marked the shields with a sign "denoting Christ". Lactantius describes that sign as a "staurogram", or a Latin cross with its upper end rounded in a P-like fashion. There is no certain evidence that Constantine ever used that sign, rather than the better known Chi-Rho sign described by Eusebius.

===Eusebius===

In Eusebius' later ''Life of Constantine'', Eusebius gives a detailed account of a vision and stresses that he had heard the story from the emperor himself.<ref group="note">Of the miracle, Eusebius wrote in the ''Vita'' that Constantine himself had told him this story "and confirmed it with oaths," late in life "when I was deemed worthy of his acquaintance and company." "Indeed," says Eusebius, "had anyone else told this story, it would not have been easy to accept it."</ref> According to this version, Constantine with his army was marching somewhere<ref group="note">Eusebius doesn't specify the actual location of the event, but it clearly isn't in the camp at Rome.</ref> when he looked up to the sun and saw a '''"cross of light"''' imposed above it, and with it, the Greek words: '''"[[w:In hoc signo vinces|Εν Τουτω Νικα]]"''' (Latin:''' ''in hoc signo vinces'' — "In this sign, conquer"'''). Not only Constantine, but the whole army saw the miracle. At first he was unsure of the meaning of the apparition, but the following night he had a dream in which [[Jesus Christ|Christ]] explained to him that he should use the sign against his enemies.