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sacred military standard of the Christian Roman emperors, first used by Constantine I in the early part of the 4th century AD. The labarum-a Christian version of the vexillum, the military standard used earlier in the Roman Empire-incorporated the Chi-Rho, the monogram of Christ, in a golden wreath atop the staff. The flag was made of purple silk (purple dye being at this time a rarity derived from a shellfish of the genus Murex) richly embroidered with gold. Although usually suspended from a horizontal bar, it appears to have been displayed occasionally by fastening one of its sides to its staff. In the Middle Ages the pastoral staff of a bishop often had attached to it a small purple scarf known as the vexillum, supposedly derived from the labarum.