of Adventure. Microsoft CEO Bill Gates was a fan of Zork, but by the time he heard of the proposal, Infocom was in negotiations with another publisher, Personal Software, one of the first professional software publishing companies. Personal Software agreed to publish the game in June 1980, sending the company an advance payment. Zork: The Great Underground Empire, also known as ZorkI or just Zork, was published for the TRS-80 in December 1980. Since Personal Software declined to publish the 1979 PDP-11 version of the game, Infocom sold some copies earlier in the year after announcing to PDP-11 user groups. Lebling recalled about twenty floppy disk copies sold directly with Anderson's typewritten manual. By the end of 1980, an Apple II version of ZorkI was completed and sold through Personal Software. Infocom began receiving requests for hints and maps as predicted, and Berez began handling map and poster orders while Dornbrook wrote customized hints for players; in September 1981 he founded the Zork Users Group as a separate company to handle all mail order sales and hint requests. Infocom eventually produced hint booklets with progressive answers to questions written in invisible ink, branded as InvisiClues. Meanwhile Lebling worked on converting the second half of Zork into ZorkII, but in the process thought up several new puzzles for the game. Although as late as December 1980 he told Byte that it would be a two-part game, it soon became clear that the second half would not fit into the allotted space. As a result, the game was split again into ZorkII: The Wizard of Frobozz and ZorkIII: The Dungeon Master. According to Lebling, splitting the game into episodes led to different atmospheres: ZorkI was focused on exploration and Adventure-style gameplay, II had more of a focus on plot and added magic spells to the base game, and III was less straightforward, with time-sensitive aspects. Marc Blank constructed ZorkIII and added gameplay changes such as the modified point system to move the game away from straightforward dungeon exploration. Zork II was offered to Personal Software in April 1981 and the contract was signed in June, but Infocom grew wary of continuing this relationship. The Infocom team felt that Personal Software was not advertising ZorkI very strongly, and did not seem excited about Infocom's plans for ZorkIII and other planned text adventure games such as Deadline and Starcross. Personal Software soon stopped publishing