Also called jūgan 銃眼 (loophole). A gun loophole or gun port. An opening through which to shoot a firearm from inside the castle. Often square or circular, although some are triangular, pentagonal, or hexagonal. The loopholes have an hourglass-shaped cross-section; they are narrow in the center of the wall and splay out at each side. This allows the greatest flexibility of fire with the smallest possible opening for an enemy to target.
Standard measurements: for triangular and square loopholes, one side 0.305 meter (1 shaku 一尺); for round openings the diameter is also 0.305 meter. The openings in the walls are located about 24 cm (8 sun 八寸) above the chest of a standing soldier, enabling him to readily assume position to fire. Loopholes designed for the use of arrows are called *yazama 矢狭間. See also *sama 狭間.