A buttress is an arrangement of stone or bricks projecting from the face of a wall to provide it with sideways support. A flying buttress is one in which the buttress does not make contact all the way up the wall.

In the construction of cathedrals, large tower-like "piers" (sometimes also referred to as buttresses) were erected next to the cathedral walls. Flying buttresses then extended from these piers to the cathedral itself--thick, sloping, half-arch-like structures. With this design, the flying buttresses provide a upward and inward force on the columns of the cathedral. Inside the cathedral, the roof is a vault, with ribs extended along the vault for support. The upward, inward force of the flying buttress supports the downward force of the ribbed vaults.

Flying buttresses are part of the Gothic style of architecture, born in France during the twelfth century. It was a time in which churches and cathedrals were centerpieces of society, often spectacular and beautiful and at the forefront of architectural development. At first the support piers were internal or inside the structure's walls. But competition for the highest vaulting lead to piers of great size, which not only had to support the weight of the vaulting but also support the tremendous sideways forces coming from such great heights. This required the piers to be thick at the bottom, and eventually they were simply placed outside the walls, to which the flying buttresses were attached. In this way the spherical vaulting was kept from pushing apart from its own weight.

The first appearance of rows of fully-developed flying buttresses was at the cathedral known as Cluny III in France around 1130 A.D. Flying buttresses were not only functional, but, as the Gothic architectural style progressed, were often elaborate and majestic when set in rows alongside a cathedral, conveying an image of delicacy. When it was found that if a pier was weighted on top it did not have to be as massive at the base, elaborate pinnacles came to top the piers, ornamental in appearance but mostly serving as additional weight.