FTP allows "experimental" commands, whose names begin with "X". If these commands are subsequently adopted as standards, there may still be existing implementations using the "X" form. At present, this is true for the directory commands:
RFC-959 "Experimental" MKD XMKD RMD XRMD PWD XPWD CDUP XCUP CWD XCWD
All FTP implementations SHOULD recognize both forms of these commands, by simply equating them with extra entries in the command lookup table.
A User-FTP can access a server that supports only the "X" forms by implementing a mode switch, or automatically using the following procedure: if the RFC-959 form of one of the above commands is rejected with a 500 or 502 response code, then try the experimental form; any other response would be passed to the user.