The idiom "以牙还牙" (yǐ yá huán yá) is formed through a symmetrical, parallel structure that literally translates as "use tooth to repay tooth," creating a powerful metaphorical meaning of retaliating in kind. Grammatically, it follows a classic Chinese rhetorical pattern where the preposition "以" (using) sets the means, "牙" (tooth) is the object employed, "还" (to repay/return) is the action, and the final "牙" is the object returned, establishing a direct, mirror-image consequence. This formation originates from ancient legal and philosophical concepts of retaliatory justice, succinctly encapsulating the principle of equivalent retribution—that one should repay an injury with an identical action. The repetition of "牙" and the balanced four-character format make it phonetically memorable and conceptually stark, perfectly conveying the idea of "an eye for an eye."