Embrasure
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The term embrasure, in architecture, refers to the opening in a crenellation or battlement between the two raised solid portions or merlons, sometimes called a crenelle; also to the splay of a window. The purpose of embrasures is to allow weapons to be fired out from the fortification while the firer remains under cover.
The etymology of embrasure expresses widening.
A loophole, arrow loop or arrow slit is a similar concept, but passes through a solid wall and was originally for use by archers. By the nineteenth century, a distinction was made between embrasures being used for cannon, and loopholes being used for musketry. In both cases, the opening was normally made wider on the inside of the wall than the outside. The outside was made as narrow as possible (slightly wider than the muzzle of the weapon intended to use it) so as to afford the most difficult possible shot to attackers firing back, but the inside had to be wider in order to enable the weapon to be swivelled around so as to aim over a reasonably large arc.
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A distinction was made between vertical and horizontal embrasures or loopholes, depending on the orientation of the slit formed in the outside wall. Vertical loopholes—which are much more common—allow the weapon to be easily raised and lowered in elevation so as to cover a variety of ranges easily. However to sweep from side to side the weapon (and its firer or crew) must bodily move from side to side to pivot around the muzzle, which is effectively fixed by the slit. Horizontal loopholes, on the other hand, facilitate quick sweeping across the arc in front, but make large adjustments in elevation very difficult. They were usually used in circumstances where the range was very restricted anyway.
Another variation had both horizontal and vertical slits arranged in the form of a cross, and was called a crosslet loop or an arbalestina since it was principally intended for arbalestiers (crossbowmen). In the sixteenth and seventeenth centuries, after the crossbow had become obsolete as a military weapon, crosslet loopholes were still sometimes created as a decorative architectural feature with a Christian symbolism.
Includes original text from 1911 Encyclopaedia Britannica.