An embrasure is the opening in a battlement between the two raised solid portions, referred to as crenel or crenelle in a space hollowed out throughout the thickness of a wall by the establishment of a bay. This term designates the internal part of this space, relative to the closing device, door or window. In fortification this refers to the outward splay of a window or arrowslit on the inside.
In ancient military engineering, embrasures were practiced in the towers and the walls, in particular between the merlons and the battle The loophole, arrow loop or arrowslit passes through a solid wall is thus an embrasure of shooting order to allow archer or gunner weapons to be fired out from the fortification while the firer remains under cover.
This type of opening was flared inward, that is the doorway was very narrow on the outside, but wide on the inside, so that the archers had free space of movement and aiming, and that the attackers have as much difficulty as possible to reach them. There are embrasures especially in fortified castles and bunkers. The generic term of loophole is gradually abandoned because of its imprecision, in favor of those more precise of archer, crossbowman, gunner archer. The splay of the wall on the inside provides room for the soldier and his equipment, and allows them to get as close to the wall face and arrow slit itself as possible. Examples of deep embrasures with arrowslits are to be seen at Aigues-Mortes and Château de Coucy, both in France.
With the appearance of firearms, the embrasure designated more specifically the opening made in a fortified structure to allow the firing of these weapons. In modern architecture, the embrasures are provided during construction because they are intended to receive a door or a window. These are not openings made after construction.
"Archimedes had had the walls pierced with large numbers of loopholes at the height of a man, which were about a palm's breadth wide at the outer surface of the walls. Behind each of these and inside the walls were stationed archers with rows of so-called "scorpions", a small catapult which discharged iron darts, and by shooting through these embrasures they put many of the marines out of action."
However, the invention was later forgotten until reintroduced in the 12th century.
By the 19th 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.
A distinction was made between horizontal and vertical 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, or where rapid cover of a wide field of arc was preferred.
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.
A stepped embrasure was often utilized on pillbox bunkers of the 20th century, allowing for a relatively wide field of fire compared to a traditional embrasure while also minimizing the shot trap phenomenon created by the sloped opening. A series of perpendicular "steps" tapering to the gun port ensured that any incoming fire would be stopped by a vertical impact and not funneled inward towards the slit. In the 19th century each step was known as a Redent.
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