The pinna is the flap of skin surrounding our ears. Reflected sound off the pinna combines with the direct sound into the ear to create high frequency comb-filtering effects (typically above 6kHz). These effects change as a function of angle of arrival, so that each angle of arrival has a unique sound quality. Our brain uses this quality as one of the ways to localize sound at each ear individually. The effect seems most persuasive in the vertical realm, so it is reasonable to hypothesize that we localize horizontally mostly by time difference while in the vertical axis the pinna effect is used more.