BUTTRESS, in architecture, a structure of masonry used to resist the thrust of an arch or vault. It takes the form of a great proportion ate thickening of the walls at the point where the thrust affects the wall, the thickness some times increasing until the mass of masonry is set across the general direction of the wall. Thus in the developed Gothic style it nearly re places the wall, because all the space between buttress and buttress is occupied by a great win dow. In the case of an archway in a single wall it often happens that the two sides or outer edges of the wall are carried up in such a way that they are spread wider toward the base and approach one another at the top, by means of certain offsets or steps, and these extensions of the wall are called buttresses, although they are mere widenings of the wall. In like manner some English Gothic church towers have curious diagonal spurs projecting on the four corners, in the form of short pieces of wall built on a prolongation of a diagonal of the square plan, and these are considered as buttresses, although they have very rarely any thrust to resist, be cause the tower is not often occupied by vaulted chambers, and because the meeting of the two walls would provide sufficient masonry for the practical purpose. It is a vice in that style that these considerations are lost sight of.
Historically, the real buttress begins to show itself in Romanesque work along the walls of the aisles, and is at first a slight projecting pilaster-like thickening of the wall, or a rounded projection like an engaged shaft of a column.
These are called by special names, as buttress pier, pilaster-strip, etc. They were very inade quate for their purpose (see ROMANESQUE ARCHITECTURE) and their presence shows the uneasiness of the early builders in try ing to dispense with the precautions taken by their masters, the Romans of the empire, and their hesitation in building what alone would do the work. As the vaulting within became concentrated on certain points, when groined vaults superseded barrel-vaults for the aisles, the need of the buttress became more evident, and in some Romanesque churches they have been built up afterward, the walls being stayed up with great cost and trouble after they had begun to spread under the thrust of the vault. It was not until the ribbed vault came in and the Gothic style came into being that the buttress took its permanent shape of a piece of wall, Chin in comparison to projection; that is to say, having by far its greatest dimen sion in the direction of the thrust of the vault and therefore at right angles with the wall of the church. Except in modern Gothic exterior buttressing is seldom employed nowadays. It is, however, employed in railway stations and factories, where strong vibration of the floors necessitates the reinforcement of the walls at regular intervals. See FLYING BUTTRESS.