Flying Buttresses. They look lovely, but actually play a large role in keeping the structure steady.
Clerestory, flying buttress, Chartres Cathedral- Chartres, Eure-et-Loire, France
DIAGRAM: illustrating "flying buttresses". The flying buttresses on Chartres are rather modest compared to later cathedrals.
Flying butresses at St. Vitus Cathedral in Prague
02.11 Gothic Architecture - Flying Buttresses(ArtHistory2)
Flying buttress typical of Gothic architecture | Washington National Cathedral...
beautiful flying buttresses at Notre Dame in Paris
All sizes | Reims Cathedral. Flying Buttresses, via Flickr.