1911 Encyclopædia Britannica/Bastion
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Bastion (through the Fr. from the late Lat. bastire, to build), a work forming part of a line of fortifications. The general trace of a bastion is similar to an irregular pentagon formed by a triangle and a narrow rectangle, the base of the triangle coinciding with the long side of the rectangle. The two sides of the triangle form the “faces” of the bastion, which join at the “salient” angle, the short sides of the rectangle form the “flanks.” Bastions were arranged so that the fire from the flanks of each protected not only the front of the curtain but also the faces of the adjacent bastions. A “tower bastion” is a casemated tower built in bastion form; a “demi-bastion” is a work formed by half a bastion (bisected through the salient angle) and by a parapet along the line of bisection; a “flat bastion” is a bastion built on a curtain and having a very obtuse salient angle.