The author missed one: Wordstar<p>See pg. 39 of this .pdf file <a href="http://bitsavers.trailing-edge.com/pdf/microPro/Wordstar_3.3/Wordstar_3.3_Reference_Manual_1983.pdf" rel="nofollow">http://bitsavers.trailing-edge.com/pdf/microPro/Wordstar_3.3...</a><p>"X" is used as the "exit wordstar" command menu option.<p>This was WordStar 3.3, in 1983. WordStar existed for far earlier than 1983, and likely always used "X" as the "exit wordstar" command.