Sonic Generations is a 2011 platform video game developed by Sonic Team and published by Sega for the PlayStation 3, Xbox 360, Nintendo 3DS, and Microsoft Windows. Produced in commemoration of the 20th anniversary of the Sonic the Hedgehog series, the game follows Sonic and his sidekick Tails as they form an alliance with their past selves. It features two gameplay styles: "Classic", which plays from a side-scrolling perspective like that of the original Sega Genesis Sonic games, and "Modern", 3D levels similar to those in Sonic Unleashed (2008) and Sonic Colors (2010). Development of Sonic Generations began following the completion of Unleashed. Sonic Team sought to re-imagine the most popular aspects of the franchise in high-definition, and developed the game using the Hedgehog Engine. Each location and many bosses in the game are previously seen in an earlier entry in the series, with the game including numerous other references to past entries. Devil's Details and Dimps helped create the Windows and 3DS versions, respectively. The Windows version is noted for its active modding scene, where a dedicated community creates new gameplay mechanics, levels, and assets for the game. The game received positive reviews from critics and was a commercial success, selling 1.85 million copies by May 2012. Reviewers found its visuals, audio, and gameplay to be highlights, and called it a good tribute to the franchise. It also received some criticism, mostly for its occasional frame rate and control problems. Reception to the 3DS version was mixed; reviewers criticized its short length and design, and deemed it worse than Dimps' previous Sonic games. The Classic iteration of Sonic introduced in the game has continued to make appearances throughout the franchise.