Embrasure

The term embrasure (/ɪmˈbreɪʒər/) comes from French (French pronunciation: [ɑ̃bʁazyʁ]), and is described as a hole in a parapet through which cannons are laid to fire into the moat or field.

The invention of the arrowslit is attributed to Archimedes during the siege of Syracuse in 214-212 BCE:

From Polybius's (c. 200-118 BC) The Histories (Book VIII, Ch. 5): "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 cannons, 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 opening formed in the outside wall. A vertical loophole, which was much more common, allows the weapon to be easily raised and lowered in elevation to cover a variety of ranges easily. To sweep from side to side, however, 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 width of the opening. A horizontal loophole, on the other hand, facilitates quick sweeping across the arc in front, but makes large adjustments in elevation very difficult. These were usually used in circumstances where the range was very restricted or where rapid cover of a wide field was preferred.

Another variation of the embrasure featured 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 16th and 17th centuries, after the crossbow had become obsolete as a military weapon, crosslet loopholes were still sometimes created as a decorative architectural feature with Christian symbolism.

A stepped embrasure was often utilised on pillbox bunkers of the 20th century. This allowed for a relatively wide field of fire compared to a traditional embrasure while also minimising the shot trap result created by the sloped opening. A series of perpendicular "steps" that tapered to the gun port ensured that any incoming fire that struck the inward-facing surface of a step would be stopped or deflected laterally by the outward-facing surface of the step and not funnelled inward toward the gunner. In the 19th century, each step was known as a 'redent', based on Old French and Latin for 'double notching' or 'like the teeth of a saw'.

On warships of the late 1860s and onwards to the First World War, the sides of a ship's hull might be recessed in locations near to a gun mounted in the hull. These recesses were also termed embrasures and were intended to allow a wider arc of fire than a standard broadside arrangement would otherwise permit. Central-battery ironclads like HMS Hercules featured such embrasures for fore and aft fire from the amidships battery's end guns. Later ironclads like HMS Alexandra featured embrasures which were 'open to the sky' in that they had no overhanging structure above the recessed area. This 'open-topped' embrasure became the standard arrangement for many cruisers and battleships of the pre-WW1 period. Once hull-mounted broadside weapons disappeared from the world's navies, so did the term.