Chi Ro

Chi Ro — one of the oldest Christogram (or several letters connected compositionally as a symbol of Jesus Christ in the form of an abbreviation) used by Christians.

Chi rho was created by superimposing the first two Greek letters chi "Χ" and Rho "Ρ", the Greek word for Christ  CHRIST , resulting in a monogram.

source wikipedia.pl

The Chi-Ro symbol was also used by pagan Greek writers to denote places of high value or importance in the fields.

The Chi-Ro symbol was used by the Roman emperor Constantine I as a vexillum, known as Labarum (the banner of the Roman legions, used only when the emperor was with the army).