Labarum

The labarum (Greek: λάβαρον) was a vexillum (military standard) that displayed the "Chi-Rho" symbol ☧, formed from the first two Greek letters of the word "Christ" (Greek: ΧΡΙΣΤΟΣ, or Χριστός) — Chi (χ) and Rho (ρ). It was first used by the Roman emperor Constantine I. Since the vexillum consisted of a flag suspended from the crossbar of a cross, it was ideally suited to symbolize the crucifixion of Christ.