as the Chi Rho, the symbol that the emperor Constantine I saw in the sky before the Battle of the Milvian Bridge in 312. First two letters in Christ in Greek, therefore symbolic of Christianity. See Labarum.
Windows XP operating system. The official Microsoft explanation for this name is a derivation from "experience"; another notes that "Cairo" was Microsoft's codename for an object-oriented operating system that would eliminate completely MS-DOS code from Microsoft Windows; Windows XP (chi-rho) completes the process.
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