One theory says this practice came from knights lifting their visors when greeting a friend
Freed from his helm, John finally lifted his head to survey the ravaged ... "Your 
brother writes that his wife is with child," said the Duke, .... As one the knights 
swore their oaths of fealty, that they would ... Sir Gregory tilted his head in 
greeting to Sherlock as well, but ... We travel, fight, and practice together.