One theory says this practice came from knights lifting their visors when greeting a friend
[DOC] [TLE] Mr. President, don't salute the troops this 4th of July | Foreign Policy [PAR] Jul 3, 2012 ... Paul McLeary | 1 day ago ..... For much of the Cold War, there was an unbroken 
line of presidents who ... believe that it originated in the need for armor-clad 
knights to have a ... otherwise wield a sword  and lift his visor, simultaneously 
making ...