I've heard a few theories, one is its to allow the door to breathe but i was always taught if you cant see it dont paint it (unless its for protection). I always check the viewing angle when coming down stairs and if you can see the top edge i paint it.
Interesting point there fellow decs. Taught by old veterans back in the late 60s and early 70s, a swift clip around the ear would be given if you had failed to paint the top and bottom of any door that faced outdoors.
( Hinges were removed and oiled while the door was off )
This was to prevent the door from swelling at certain times of the year, however it was still in the days of coal fires and paraffin heaters.
Central heating was in short supply in them days.
As you said dcdec, any door top seen from a higher point was expected to be painted but not the bottom. One other exception was kitchen cuboard doors and this was so that they were easier for scullery maids to wash down and keep clean from greasy kitchen smuts !!!
I must admit I vary rarely have to 'ease' doors before painting doors these days, central heating seems to have cured this little problem.
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