These materials were very difficult for others to copy or otherwise reproduce, and many included information that was essential to completing the game. Seeing the potential benefits of both aiding game-play immersion and providing a measure of creative copy-protection, in addition to acting as a deterrent to software piracy, Infocom and later other companies began creating feelies for numerous titles. In 1987, Infocom released a special version of the first three "Zork" titles together with plot-specific coins and other trinkets. This concept would be expanded as time went on, such that later game feelies would contain passwords, coded instructions, page numbers, or other information that would be required to successfully complete the game.