OK, so it's better to have heating registers under windows (where they'll
hopefully combat the cold air coming off the glass) in a home and return
vents more central - that makes sense.

What about large picture windows, though? Is it better to have one central
register below the middle of the window, or two smaller ones to either
side?

I was just thinking that if the curtains are shut, most of the cold
air is going to be coming around them at the edges rather than in the
middle (and if the curtains are open, two side vents should still
hopefully help at least a little)

cheers

Jules