George Frideric Handel's Hallelujah Chorus from his oratorio The Messiah.
It is common in performances of this piece for the audience to stand while the Hallelujah is being sung. This is reportedly because at the oratorio's premier in 1742 the present King George II stood up as this section started. Royal protocol dictates that all others must stand and thus a tradition was born (apparently!) His majesty could have stood for various reasons namely he was so moved by the music he wanted to give his appreciation or that he was signifying his obedience to the ultimate 'King of Kings' as the work described. Of course it could have been that his leg had fallen asleep!
