A crossbow is a ranged weapon using an elastic launching device consisting of a bow-like assembly called a prod, mounted horizontally on a main frame called a tiller, which is hand-held in a similar fashion to the stock of a long gun. Crossbows shoot arrow-like projectiles called bolts or quarrels. A person who shoots crossbow is called a crossbowman, an arbalister or an arbalist (after the arbalest, a European crossbow variant used during the 12th century).
Crossbows and bows use the same elastic launch principles, but differ in that an archer using a bow must draw-and-shoot in a quick and smooth motion with limited or no time for aiming, while a crossbow's design allows it to be spanned and cocked ready for use at a later time and thus affording the wielder unlimited time to aim. When shooting a bow, the archer must first fully perform the draw, holding the string and arrow while pulling them back with arm and back muscles, and then either immediately loose without a period of aiming, or hold that form while aiming. When using the heavy bows suitable for warfare, both actions demand some physical strength. As such, the accurate and sustained use of a bow in warfare takes much practice.
Crossbows avoid these potential problems by having trigger-released cocking mechanisms to maintain the tension on the string once it has been spanned – drawn – into its ready-to-shoot position, allowing a crossbow to be carried cocked and ready and affording its user time to aim it. This also allows for crossbows to be operated in succession by groups of people, with one person operating a cocked crossbow while others reload and ready them. Crossbows are spanned into their cocked positions using a number of techniques and devices, some of which are mechanical and employ gear and pulley arrangements – levers, belt hooks, pulleys, windlasses and cranequins – to overcome very high draw weight. These potentially achieve better precision and enable their effective use by less familiarised and trained personnel compared to the training and practice necessary to become proficient with the English longbow or the bows of steppe nomads.
These advantages for the crossbow are somewhat offset by the longer time needed to reload a crossbow for further shots. Crossbows with high draw weights require sophisticated systems of gears and pulleys to operate that are awkward and slow to employ on the battlefield. Medieval crossbows were also very inefficient, with short shot stroke lengths from the string lock to the release point of their bolts, along with the slower speeds of their steel prods and heavy strings, despite their massive draw weights compared to bows. Modern crossbow designs overcome these shortcomings.
The earliest known crossbows were invented in ancient China in the first millennium BCE and brought about a major shift in the role of projectile weaponry in wars, especially during Qin's unification wars and later the Han campaigns against northern nomads and western states. The medieval European crossbow was called by many names, including "crossbow" itself; most of these names derived from the word ballista, an ancient Greek torsion siege engine similar in appearance but different in design principle. The Greek crossbow was likely invented independently of the Chinese crossbow.
In modern times, firearms have largely supplanted bows and crossbows as weapons of war, but crossbows remain widely used for competitive shooting sports and hunting, and for relatively silent shooting.
A crossbowman is sometimes called an arbalist, also spelled arbelist, or historically an arbalister. An extensive list of archaic words for medieval crossbowmen is given by Payne-Gallwey. Richardson, in his 1839 dictionary, did not make specific reference to the crossbow in his definition of arbalist: "One who casts or shoots from a bow." Hansard (1841) used the word arbalister for a cross-bowman (sic), the same usage as Webster and Johnson who reserved the word arbalist for the crossbow itself. Smith uses arbalist to describe a maker of crossbows.
Arrow, bolt and quarrel are all suitable terms for crossbow projectiles, as was vire historically.
The lath, also called the prod, is the bow of the crossbow. According to W. F. Peterson, prod came into usage in the 19th century as a result of mistranslating rodd in a 16th-century list of crossbow effects.
The stock (a modern term derived from the equivalent concept in firearms) is the wooden body on which the bow is mounted, although the medieval tiller is also used.
The lock refers to the release mechanism, including the string, sears, trigger lever, and housing.
A crossbow is essentially a bow mounted on an elongated frame (called a tiller or stock) with a built-in mechanism that holds the drawn bow string, as well as a trigger mechanism, which is used to release the string.
The Chinese trigger was a mechanism typically composed of three cast bronze pieces housed inside a hollow bronze enclosure. The entire mechanism is then dropped into a carved slot within the tiller and secured together by two bronze rods.
The string catch (nut) is shaped like a "J" because it usually has a tall erect rear spine that protrudes above the housing, which serves the function of both a cocking lever (by pushing the drawn string onto it) and a primitive rear sight.
It is held stationary against tension by the second piece, which is shaped like a flattened "C" and acts as the sear. The sear cannot move as it is trapped by the third piece, i.e. the actual trigger blade, which hangs vertically below the enclosure and catches the sear via a notch.
The two bearing surfaces between the three trigger pieces each offers a mechanical advantage, which allow for handling significant draw weights with a much smaller pull weight.
During shooting, the user will hold the crossbow at eye level by a vertical handle and aim along the arrow using the sighting spine for elevation, similar to how a modern rifleman shoots with iron sights.
When the trigger blade is pulled, its notch disengages from the sear and allows the latter to drop downwards, which in turn frees up the nuts to pivot forward and release the bowstring.