Sonic the Hedgehog platforming of the main levels. The 16-bit "Sonic the Hedgehog " consisted of a giant rotating maze, which many considered a major technical achievement. The most common special stages, however, were segments with the character running through a long tunnel to collect certain items. 3D "collect item" levels, as in "Sonic 3" and "Sonic & Knuckles", used the same perspective but had Sonic collecting all the blue-colored orbs on the surface of a giant sphere. Finally, "Sonic Advance 2" employed a 3D ring-collecting stage, and "Sonic Chaos" ("Sonic and Tails" in Japan) used a variety of gimmicks for its levels.
Sonic the Hedgehog forms, or both. The methods used to acquire the Emeralds and the end-results after collecting them differ between games in the series. Most early games require the player to find the emeralds in Special Stages, while some modern games implement the emeralds as a plot element. In certain games, such as "Sonic R" and the 8-bit versions of "Sonic the Hedgehog" and "Sonic the Hedgehog 2", the player is required to find the Chaos Emeralds within the zones themselves. The Master Emerald was introduced in "Sonic & Knuckles" as a plot element. It resides in a shrine on Angel Island