If you book the seat at the left side of a screen, then the person sitting at the left most corner will get the left side of his arm rest and the full row gets the left as their arm rest. For that side right corner seat person gets both the arm rest.
If you book the seat at the right side of a screen, then the person sitting at the right most corner will get the right side as his arm rest and the full row gets the left as their arm rest. For that side left corner seat person gets both the arm rest.
