Kludge line
Hidden metadata lines in FidoNet messages, each starting with a Ctrl-A character that mail readers normally kept out of sight.
FidoNet's original message header had no room for zones, points or threading. Rather than change the format and break every program, developers tucked the extra data into the message body on lines beginning with byte 0x01. Software that understood them read them; older software was supposed to ignore them. The name was an honest admission of what it was.
Over time the kludges became the most important part of a message, at least for the machines. INTL carried zone addresses for netmail, MSGID and REPLY made threading possible, PID named the program and TZUTC the time zone. Adding headers without breaking old readers is, broadly, what email's X- headers do too.