POSIX, the Portable Operating System Interface published as IEEE 1003 and ISO/IEC 9945, is the quiet standard that lets software written once run across Linux, macOS, commercial UNIX, and embedded systems. This UpLaw reference explains what POSIX specifies and who maintains it (the IEEE, ISO/IEC, The Open Group, and the Austin Group), then treats the standard as what it really is for counsel: a single legal object with five faces. It works through whether a published interface can be owned under copyright after Google LLC v. Oracle America, Inc.; how standard-essential patents and FRAND commitments cap patent leverage and get priced by courts; the antitrust exposure of the competitor-filled rooms where standards are written; the open-source license obligations that ride on every real implementation; and what changes when a government makes conformance a procurement command. The through-line is the interface/implementation distinction, and the payoff is seeing all five faces at once instead of one at a time.