The Chi Rho 
/ˈkaɪ ˈroʊ/ 
also known as chrismon is one of the earliest forms of christogram, formed by superimposing the first two capital letters —chi and rho (ΧΡ)—of the Greek word ΧΡΙΣΤΟΣ (Christos) in such a way that the vertical stroke of the rho intersects the center of the chi.