A key, or in anatomy, the clavicle (collarbone); also used in medieval texts as a reference work or explanatory guide.
From Latin clavis (key). The Romans named the collarbone clavis because of its key-like shape. Medieval scholars adopted the term for reference works that 'unlocked' the meaning of texts, like a key.
The clavicle is literally called a 'key bone' because Romans thought it looked like a key—and this name has stuck for 2,000 years across dozens of languages. Ancient people were surprisingly poetic when naming bones!
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