Sonic & Knuckles (Japanese: ソニック&ナックルズ, Hepburn: Sonikku to Nakkuruzu) is a 1994 platform video game developed by Sonic Team and published by Sega for the Sega Genesis/Mega Drive. Released on October 18, 1994, it was the fourth installment in the main Sonic the Hedgehog series. The game is a direct sequel to Sonic the Hedgehog 3, and follows both Sonic the Hedgehog and Knuckles the Echidna in their respective quests to save Angel Island; Sonic tries to prevent Dr. Robotnik from re-launching his orbital weapon, the Death Egg, while Knuckles scuffles with Robotnik's minion, EggRobo.

Sonic & Knuckles utilizes "lock-on technology" that allows the game to access data from both Sonic the Hedgehog 2 and Sonic the Hedgehog 3 so that elements of both Sonic & Knuckles and the locked-on game are combined. During the development of Sonic 3, it and Sonic & Knuckles were intended to be a single story within the same cartridge. However, due to time constraints and the fact that a large memory capacity cartridge would have been expensive, Sega split the game into two separate installments.

The knuckles are the joints of the fingers which are brought into prominence when the hand is clenched and a fist is made. The word is cognate to similar words in other Germanic languages, such as the Dutch "Knokkel" (knuckle) or German "Knöchel" (ankle), i.e., Knöchlein, the diminutive of the German word for bone (Knochen). Anatomically, it is said that the knuckles consist of the metacarpophalangeal (MCP) and interphalangeal (IP) joints of the finger. The knuckles at the base of the fingers may be referred to as the 1st or major knuckles while the knuckles at the midfinger are known as the 2nd and 3rd, or minor, knuckles. However, the ordinal terms are used inconsistently, and can be found referring to any of the knuckles.

The physical mechanism behind the popping or cracking sound heard when cracking joints such as knuckles is still uncertain, although it is widely believed to be caused by synovial fluid filling the vacuum left by the joint's displacement.

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Clenched fist with knuckles (encircled in red)

Knuckles the Echidna (ナックルズ・ザ・エキドゥナ, Nakkuruzu za Ekiduna) is a fictional character in Sega's Sonic the Hedgehog series of video games. He is a red anthropomorphic echidna who is determined and serious but sometimes gullible. He can glide and climb up walls, and is a powerful fighter due to his spiked hands. He serves as the guardian of the Master Emerald, a huge gemstone that controls the series' integral Chaos Emeralds.

Knuckles debuted as an antagonist in Sonic the Hedgehog 3 in 1994; Doctor Eggman tricks him into opposing Sonic and Tails. He first became playable in Sonic & Knuckles later that year; he learns of Eggman's trickery and teams up with Sonic during that game's events. Since then he has appeared in dozens of playable and non-playable roles, as well as in several series of comic books, Western animated television, and Japanese anime. He is one of the series' most popular and recognizable characters.

His reception from critics has been mostly positive; he has been praised frequently as tough and cool-looking. However, some critics have voiced disapproval at the series' extensive cast of characters and singled Knuckles out as ushering them in. His likeness has frequently appeared in Sonic merchandise.