Other single keystroke commands exist as well, such as Known rooms, which lists the rooms known to the user. This elegantly small command set made the system so usable that many daily users during Citadel’s golden era were never aware that Citadel also provided sophisticated capabilities. These are known as the “dot” commands and build logically from the set of single keystroke commands. A simple example would be the requirement to go directly to a specified room. The user would type codice_1, where the text between the brackets is user typed, while the rest is filled in by the system. A more complex example might be .Read All rooms Zmodem New messages (.RAZN), which results in all of the new messages in all of the rooms known to the user being sent to the user via the ZMODEM protocol. Filters for users, keyword searches, and other capabilities have been implemented, depending on the version of Citadel. Citadel was originally written for the CP/M operating system in 1981 by Jeff Prothero, known to the nascent Citadel world as Cynbe ru Taren (CrT). Unlike most BASIC-based BBS programs of the time, it was written in a fairly standard dialect of C known as BDS C, a compiler written and distributed by Leor Zolman. The first installation came online in December, 1981, running on a Heathkit H-89, and in its 6 month lifetime achieved immediate success. Version 2 debuted on David Mitchell's ICS BBS, and with the release of 2.11, Prothero's involvement with the project ended following a conflict centered around a user called "sugar bunny". He released the source to the public domain and it became available as a download from various systems as well as through the C Users Group. At this point, the history of Citadel becomes complex as many individuals began modifying the source to their own ends, and lacking modern distributed source tracking, innovations were never incorporated into a central source repository, as such a thing did not exist. Initially, Bruce King, David Bonn (releasing under the name Stonehenge), Caren Park, and James Shields, amongst others, picked up the opportunity in the Seattle area. The longest lived fork from the 2.10 code started in the American Midwest, when Hue White (aka Hue, Jr.) ported the code to MS-DOS and called it Citadel-86 ("C-86"). His board, Citadel-86 Test System, served not only as a discussion board and distribution center for the software, but also was the focal point for a lively Citadel-86 community in the 612 area code (the Twin Cities), which at their peak numbered roughly forty systems, and probably more than 100 over the years.