Staurogram

The staurogram (⳨), also monogrammatic cross or tau-rho,[1] is a ligature composed of a superposition of the Greek letters tau (Τ) and rho (Ρ).

[4] Ephrem the Syrian (4th century) discusses a Christian symbol, apparently combining the Tau-Rho with Alpha and Omega placed under the left and right horizontal arms of the Tau.

Ephrem says that the Tau represents the cross of Jesus (prefigured by the outstretched hands of Moses in Exodus 17:11), the Alpha and Omega signify that the crucified Christ is "the beginning and end", and the Rho, finally, signifies "Help" (βοήθια [sic]; classical spelling: βοήθεια), because of the numerological value of the Greek word being 100, represented by Rho as a Greek numeral.

[5] The two letters tau and rho can be found separately (not in ligature) as symbols already on early Christian ossuaries.

[6] Tertullian (Contra Marcionem 3.22) explains the Tau as a symbol of salvation by identification with the sign which in Ezekiel 9:4 was marked on the forehead of the saved ones.

The Staurogram, also called the Tau-Rho, and the Monogrammatic cross.
A staurogram used as τρ-ligature part of the spelling of the word σταυρον (as Ϲ⳨ΟΝ ) in Luke 14:27 ( Papyrus Bodmer XIV , 2nd century).
A solidus minted under Anastasius I Dicorus (struck in Constantinople between 507–518). On the obverse is Victory standing left, holding a staff surmounted by a staurogram.
An oil lamp with staurogram from Caesarea Maritima . Glass Factory Museum, Nahsholim , Israel.