not all of them are. Some countries have triangular flags…
If this shape has a name, I don’t know what it is.
Edit. I deleted myself. I apologize if anyone read my original answer.
I stand corrected: http://en.wikipedia.org/w/index.php?title=National_flag#Unusual_flags One is triangular. 2 are square.
Squares are rectangular.
Interestingly, the state flag of Ohio is the only non-rectangular shaped state flag in the USA.
So they can flap in a breeze really well.
@loser I thought of posting that, but it seemed to easy :)
Its a custom I guess. prolly dating why back to banners.