Thanks for that - I make a mental note each time upon boarding, and then promptly forget when I get off the plane. Each time I'm selecting a seat in the main dorm room in J I go through the same question (I much prefer the window seats, particularly on 77D). I'm coming back from JFK in a few days I'll try to remember to post. If I recall correctly, there are two places missing windows: the one in row 21 as you mention, and another somewhere else.