After finishing his game ZZT, Sweeney opted to re-use the Potomac Computer Systems name to release the game to the public in January 1991.[1][8] It was only with the unexpected success of ZZT, caused in most part by the easy modifiability of the game using Sweeney's custom ZZT-oop programming language,[9] that made Sweeney consider turning Potomac Computer Systems into a video game company.[7] ZZT was sold through bulletin board systems, while all orders were fulfilled by Sweeney's father, Paul Sweeney.[10] The game sold several thousand copies as of May 2009, and Paul Sweeney still lived at the former Potomac Computer Systems address at the time, fulfilling all orders that eventually came by mail.[7][10] The final copy of ZZT was shipped by Paul Sweeney in November 2013.[10]