OK, who's armrest is it?.

  When you go to the movies, a play, or ballgame, and you have seats as shown, who's armrest is it?  I mean, a seat shares an armrest? Two people can't 'share' an armrest.  One of the two people have to dominate it.  Are you supposed to sit with your hands in your lap and nobody use it?  Why don't they make two armrests between seats?  Maybe it's first come, first served. 
Oh well, this is just one of those little quirks I've always wondered about.  What do you do?