Flying buttresses--a Notre Dame original--hold up the high walls of this Gothic cathedral : Paris, FR
According to Wikipedia, Notre Dame was one of "the first buildings in the world to use the flying buttress (arched exterior supports)." Apparently, the original building didn't call for this feature, but stress fractures began appearing in the walls as the building grew in height. Arched exterior supports were added to hold up the walls and this eventually become a pattern in this type of Gothic architecture. Load bearing columns also assist in holding up the high ceilings that make this type of architectural style so lofty and awe-inspiring. This picture was taken from the back of the building and you can just see them peeking out above the tree line, holding up the apse of this grand cathedral.
Photo copyright: Janet M Kincaid, 1/10