Common features in Sonic the Hedgehog

Rings
One distinctive feature of Sonic games are the collectible golden rings spread throughout the levels. This gameplay device allows players possessing at least one ring to survive upon sustaining damage from an enemy or hazardous object; instead of dying, the player's rings are scattered. In most Sonic games, a hit causes the player to lose all of the rings, although in certain games a hit only costs a set number of rings such as ten or twenty. When the rings are scattered, the player has a short amount of time to recollect some of them before they disappear.

Some causes of death cannot be prevented by holding a ring, including being crushed, falling into a bottomless pit, drowning, and running out of time.

In line with many platform games, collecting 100 rings usually rewards Sonic or any other playable character with an extra life. Certain games in the series often reward the collection of 50 rings with Chaos Emeralds, access to the Special Stages in which the Chaos Emeralds may be obtained, or utilization of a character's super transformation.

Rings have had other uses in various games as well, such as currency (Sonic Adventure 2), restoring health bars, (Sonic Unleashed) or improving statistics (Sonic Riders).

Chaos Emeralds
The Chaos Emeralds are, in most games, seven emeralds with mystical powers; they are a recurring feature of Sonic games. The emeralds can turn thoughts into power, warp time and space with a technique called Chaos Control, give energy to all living things and be used to create nuclear or laser based weaponry.

They are the basis of many of the games' plots, and the player is frequently required to collect them all to fully defeat Doctor Eggman and achieve the games' "good endings", super forms, or both. The methods used to acquire the Emeralds and the end-results after collecting them differ between titles 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 and is guarded by Knuckles the Echidna, as only those of the Echidna Tribe are able to control it. The power of the Master Emerald is what keeps Angel Island afloat in the sky. It can control the power of the Chaos Emeralds, including neutralizing or amplifying their energies.

Special Stages
Usually, Special Stages were employed as a means of earning Chaos Emeralds. Special Stages usually take place in surreal environments and feature alternate gameplay mechanics to the standard 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. Since Emeralds of the 8-bit version of Sonic the Hedgehog were hidden in the main stages, the game's spring-filled Special Stages were merely used as a means of adding variety, increasing score and earning continues. Similarly, Sonic 3 & Knuckles, in addition to the main Special Stages, featured entirely optional bonus stages, one of which combined the rotating maze of Sonic the Hedgehog with the pinball gambling of Sonic the Hedgehog 2. Sonic Heroes contained an alternate Special Stage for a chance of earning additional lives.

Super transformation
A super transformation is a state certain characters go into that gives them incredible speed, near-invincibility and a change in color. In some games, characters are still vulnerable to being crushed, drowning, falling into a pit or running out of time. Depending on the type of transformation, the condition to attain such a form is an individual making contact with all of the Chaos Emeralds, the Sol Emeralds, or the Master Emerald. Super transformations first appeared in Sonic the Hedgehog 2, where Sonic transforms into Super Sonic if all seven Chaos Emeralds and fifty or more rings are held at the same time. While in Super Sonic form, one ring is lost for every second that passes, and Sonic reverts to normal if the number of rings drops to zero. In Sonic 3 & Knuckles, Tails can also transform after gaining all seven Chaos Emeralds and Super Emeralds. Sonic and Knuckles can also transform into their hyper counterparts with the Super Emeralds.

In most 3D Sonic games starting with Sonic Adventure, Chaos Emeralds are collected in non-interactive cut scenes as part of the story, with Super Sonic and other super characters only appearing in the final boss fights. Most 2D Sonic platform games, like the Advance and Rush series, have retained the gameplay-based emerald collecting, but still have Super Sonic only playable in the "Extra" boss battles. Recent games such as Sonic the Hedgehog 4, Sonic Colors,Sonic Generations,and Sonic: Lost World once again allow players to play as Super Sonic during normal stages, provided they have all the Chaos Emeralds.

Sonic the Hedgehog, Miles "Tails" Prower, Knuckles the Echidna, Metal Sonic, Shadow the Hedgehog, Blaze the Cat, and Silver the Hedgehog, are the characters that are known to be able to perform super transformations. Metal Sonic can also use the Master Emerald or Chaos Emeralds to turn Super, and Blaze, instead of using the Chaos Emeralds, uses the Sol Emeralds (her dimension's equivalent).

Item boxes
These are containers hold power-ups and appear frequently throughout the stages. An icon on each box indicates what it contains, and the player releases the item by destroying the box. In the early games, item boxes resembled television sets and could only be destroyed with an attack; in later titles, they became transparent capsule-like objects easily destroyed with one touch. Common items in boxes include rings, a barrier (or shield), invincibility, high speed (or power sneakers) and 1-ups.

Ring boxes have a ring in the middle. When opened, they add a set amount of rings to the player's total.

The barrier is a spherical energy shield which surrounds and protects the player's character from one attack; when hit, the barrier is lost instead of rings or a life. In Sonic 3, additional barriers were introduced which gave the player special abilities, such as the ability to magnetically attract rings and double jump, breathe underwater, resist fire.

Invincibility temporarily protects against damage done by enemies and obstacles, and allows the player to destroy enemies by touching them and not lose any rings. Death from crushing, falling, drowning and time-ups, however, are still possible.

High speed boxes give the player character enhanced speed for a limited time.

1-up boxes display the face of the player's character and give the player one extra life. In the event that a player loses a stage, this enables the player to restart the level at the starting point, or, if one has been passed, close to the last checkpoint. Multiple lives can be collected, generally up to 99.

Other item boxes featured include a box with Robotnik's face on it, which cause damage to whoever opens it, a "teleport box" (Sonic 2's 2-player mode only), which swapped both players' positions, and a skateboard box (Sonic Generations only), which allows the player to ride a skateboard for a short time.

Giant Rings
Giant rings (Warp Rings) were featured in a few Sonic games, mainly from the 16-bit era. Rather than collecting them, as is the case with the smaller ones, they served as a portal to enter a Special Stage, where the player could collect one of the Chaos Emeralds. In Sonic the Hedgehog 3, if all Emeralds have already been found, touching them rewards the player fifty rings. In most games since Sonic Adventure 2, these giant rings serve as the end level marker, which ends the level upon touching it.

Checkpoints
Checkpoints are items placed throughout the stages in Sonic games which serve mainly as progress markers. If the player runs through one, their progress through a level is "saved". If the player then loses a life on the same stage, they will start over at the last checkpoint passed. Checkpoints also serve other uses in various games, such as entering Special Stages in Sonic the Hedgehog 2, and leveling up in Sonic Heroes. In the 2D games, checkpoints take the appearance of posts, while in 3D games they are either small gates or pads on the ground.

Springs
Springs are a staple in the Sonic series. They are scattered throughout the levels and serve to catapult the player at high speeds in a particular direction. Sometimes they allow the player to proceed further in the level, while other times they are used to hinder the player, usually by sending Sonic towards a dangerous area. Sonic Unleashed in particular features springs with Dr. Eggman's face on them that launch Sonic towards danger or hinder his progress. Springs serve as one of Sonic's special moves in Super Smash Bros. Brawl.

Animals
In addition to anthropomorphic animals, the Sonic universe is also populated with more conventional animals. These small animals are often used by Dr. Eggman as "organic batteries" to power his robot armies. The trapped animals can usually be freed by hitting the robot and destroying its metal case. In Sonic Adventure and Sonic Adventure 2, the small animals can be given to Chao, altering their appearance and attributes.

A Flicky is one of the most prominently used animals in Eggman's experiments. This small bird's first appearance in a game predates Sonic. Flicky and its respective game were alluded to in Sonic 3D Blast.