Portable Archive Exchange (PAX) file created by pax, a command line utility used for creating and extracting archives; may store the archive in one of pax's several supported formats, which include bcpio, cpio, sv4cpio, sv4crc, tar, and ustar.

Sometimes the ".pax" extension is used as an alternative to the .TAR and .CPIO formats, since there were differences that evolved between the two formats as they were developed. In this manner, the pax utility can handle both formats transparently while the file extension remains neutral. NOTE: The ".pax" extension is not commonly used. Instead, archives used by pax usually retain their original extensions, such as ".tar" or ".cpio." By default, pax creates archives in the ustar format.