The vexillum was a flag-like object used as a military standard by units in the Roman army. A common vexillum displayed imagery of the Roman aquila on a reddish backdrop.
Modern reproduction of a Roman cavalry vexillum
The Chi Rho is one of the earliest forms of the Christogram, formed by superimposing the first two letters—chi and rho —of the Greek ΧΡΙΣΤΟΣ in such a way that the vertical stroke of the rho intersect
Vexillum
…termed labara (Greek: λάβαρον) after the standard adopted by the first Christian Roman emperor Constantine I replaced the usual spear point with the "Chi-Rho" symbol ☧. For example, a vexillum is used by the Legion of Mary as the term for its standards. A small version is used on the altar and a larger one…
Missorium depicting Emperor Constantine the Great's son Constantius II accompanied by a guardsman with the Chi Rho depicted on his shield (at left, behind the horse)
Coin minted during reign of Ptolemy III Euergetes in Egypt (from 246 BC to 222 BC). Chi Rho visible between bird's legs.
This coin of Crispus, son of Constantine, with a chi rho on the shield (struck c. 326) shows that the symbol mentioned by Lactantius and Eusebius was a chi rho.
The so-called "Chrismon of Saint Ambrose" (Chrismon Sancti Ambrosii), on display on the eastern wall of Milan Cathedral, a Chi-Rho combined with Alpha and Omega in a circle. According to Landulf of Milan (12th century), it was used by Saint Ambrose to introduce the catechumens to the mysteries of the Christian faith (whence it was called "oracle" or chresmos of St. Ambrose, written by Landulf as crismon, whence the later Neo-Latin term for the Chi-Rho symbol).