a) The Chi-Rho Symbol. The oldest monuments bearing this symbol date to the third century, the first being found on a burial inscription of a consul from the year 369. From the third century we have the following authentic forms: (1) the separate letters; (2) the superimposed letters; (3) the monogram cross. After the triumph of Christianity, the Chi-Rho monogram spread into all countries and found manifold use. It no longer served as a mere abbreviation of the sacred Name, but stood as a symbol for Christ the King. Artists often surrounded it with a laurel wreath or a circle. This signified Chris's dominion over the world, or His triumph over all enemies of His kingdom.
b) The IHS Symbol. The familiar abbreviation IHS is a symbol of the Name Jesus which has retained its popularity down through the centuries. It owes its spread to St. Bernardine of Siena, who had it placed on his banner, surrounded with twelve rays of the sun and surmounted by a cross. It soon became the most popular monogram for the Holy Name of Jesus. By his words St. Bernardine persuaded many priests to place the letters on altars or on the interior and exterior walls of churches. Many Italian cities responded to his efforts and put the monogram in large letters on the outer walls of their town halls, as may still be seen in Siena.
c) Ichthys. The early Church loved another monogram for our Savior's name, the widely-used ICHTHYS. Christ's full title was (in Greek): Iesous Christos Theou Yos Soter, Jesus Christ, Son of God, Savior. The initial letters of these five words form the Greek word ICHTHYS, which means fish. For this reason Christ is often pictured as a fish. Thus both the abbreviation and the picture became for the early Christians a secret symbol for the Redeemer. Tertullian presupposed popular familiarity with the fish symbol when he wrote about 200: "We (Christians) are born as little fishes in water after the model of our Ichthys Jesus Christ"