When painting a room, which surface is done first...ceiling or the walls.???

I can see that it would be easier to do the walls first as they are easier to mask
so that you can get a straight line between wall and ceiling.

But some people swear that the ceiling should be done first. Isn't it more
difficult to mask the ceiling for a straight paint line ???


When painting a wallI is a edge trimmer the best way to get a straight line,
(just the wall needs to be painted....the ceiling is fine)......

Thanks in advance for your help !!!

Peter