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Celerity BBS was a descendant of the freely distributed source of TCS BBS 1.43, and ultimately nearly completely rewritten.
It originally began as a project of Brendon Woirhaye (The Byter) and David Hicks (Moebius) in 1990 to quickly modify an existing BBS package to meet a simple organizational need (separate conferences for IBM PC users and Amiga users), and to meet the needs of high speed (9600 bit/s) communication, as most BBSes of the time could not pump data to the modem quickly enough. The I/O and display subsystems were rewritten, and the BBS package got its name.
It wasn't long before others became interested in the BBS software, and it went on to have over 500 authorized BBSes and about three times that number using it in an unauthorized manner. It was very popular in the underground pirate, hacking, and phreaking community as well as with legitimate systems, including church BBSes, non profit group BBSes, many shareware distribution systems, and a governmental BBS in Portugal.
Celerity BBS had over 50 discrete releases between 1990 and 1995, and pioneered a number of new technologies which were not commonly seen, including split screen multiuser chat (8 node maximum), a file distribution network, an early "liveupdate" system where BBS updates were distributed, a cross BBS message board and email network (CelerityNet, adopted by some non-Celerity BBS programs), QWK offline reader support, the ability to completely change the look and feel of the system, "geek speak" mode that transformed normal text to leet, and early internet FTP and Telnet connectivity.
Alacrity BBS was a premium version of Celerity BBS which was not broadly distributed. Its most notable feature was an AI sysop chat which incorporated ELIZA-style logic, sprinkled with references to current posts on the message boards and recently uploaded files.