One of the lateral ankle ligaments
Prevents inversion in neutral position

Location: "The calcaneo-fibular ligament is a ligament connecting the apex of the lateral malleolus to the posterolateral aspect of the calcaneus." (1) 

Significance: "Tenderness, swelling, and ecchymosis in the area inferior and posterior to the lateral malleolus is evidence of a calcaneo fibular ligament sprain." (1) 




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Sources:
(1) http://www.orthopaedicsone.com/display/Main/Calcaneo+fibular+ligament+Surface+Anatomy