made, over twenty years since the game's release. Custom maps are new maps that are playable by loading them into the original game. Custom levels of various gameplay types have been made, but most are in the single-player and deathmatch genres. More than 1500 single-player and a similar number of deathmatch maps have been made for Quake. Reception Sales According to David Kushner in Masters of Doom, id Software released a retail shareware version of Quake before the game's full retail distribution by GT Interactive. These shareware copies could be converted into complete versions through passwords purchased via phone. However, Kushner wrote that "gamers wasted no time hacking the shareware to unlock the full version of the game for free." This problem, combined with the scale of the operation, led id Software to cancel the plan. As a result, the company was left with 150,000 unsold shareware copies in storage. The venture damaged Quakes initial sales and caused its retail push by GT Interactive to miss the holiday shopping season. Following the game's full release, Kushner remarked that its early "sales were good — with 250,000 units shipped — but not a phenomenon like Doom II." In the United States, Quake placed sixth on PC Data's monthly computer game sales charts for November and December 1996. Its shareware edition was the sixth-best-selling computer game of 1996 overall, while its retail SKU claimed 20th place. The shareware version sold 393,575 copies and grossed $3,005,519 in the United States during 1996. It remained in PC Data's monthly top 10 from January to April 1997, but was absent by May. During its first 12 months, Quake sold 373,000 retail copies and earned $18 million in the United States, according to PC Data. Its final retail sales for 1997 were 273,936 copies, which made it the country's 16th-highest computer game seller for the year. Sales of Quake reached 550,000 units in the United States alone by December 1999. In 1997, id estimated that there may be as many as 5 million copies of Quake circulating. The game sold over 1.4 million copies by December 1997. Critical reviews Quake was critically acclaimed on the PC. Aggregating review websites GameRankings and Metacritic gave the original PC version 93% and 94/100, and the Nintendo 64 port 76% and 74/100. A Next Generation critic lauded the game's realistic 3D physics and genuinely unnerving sound effects. GamePro said Quake