The labarum was a vexillum that displayed the "Chi-Rho" symbol ☧, a christogram formed from the first two Greek letters of the word "Christ" – Chi (χ) and Rho (ρ). It was first used by the Roman emperor Constantine the Great.
A follis of Constantine (c. 337) showing a depiction of his labarum spearing a serpent on the reverse; the inscription reads SPES PVBLICA
Coin of Vetranio, a soldier is holding two labara. Notably, they differ from the labarum of Constantine in having the Chi-Rho depicted on the cloth rather than above it, and in having their staves decorated with phalerae as were earlier Roman military unit standards.
The emperor Honorius holding a variant of the labarum - the Latin phrase on the cloth means "In the name of Christ [rendered by the Greek letters XPI] be ever victorious."
Modern ecclesiastical labara (Southern Germany).
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