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 that allows the string to be released. The earliest designs featured a transverse slot in the top surface of the frame, down into which the string was placed. To shoot this design, a vertical rod is thrust up through a hole in the bottom of the notch, forcing the string out. This rod is usually attached perpendicular to a rear-facing lever called a tickler. A later design implemented a rolling cylindrical pawl called a nut to retain the string. This nut has a perpendicular centre slot for the bolt, and an intersecting axial slot for the string, along with a lower face or slot against which the internal trigger sits. They often also have some form of strengthening internal sear or trigger face, usually of metal. These roller nuts were either free-floating in their close-fitting hole across the stock, tied in with a binding of sinew or other strong cording; or mounted on a metal axle or pins. Removable or integral plates of wood, ivory, or metal on the sides of the stock kept the nut in place laterally. Nuts were made of antler, bone, or metal. Bows could be kept taut and ready to shoot for some time with little physical straining, allowing crossbowmen to aim better without fatiguing.
The bow (called the prod or lath on a crossbow) of early crossbows was made of a single piece of wood, usually
yew. Composite bows are made from layers of different material, often wood, horn, and sinew glued together and bound with animal tendon. These composite bows made of several layers are much stronger and more efficient in releasing energy than simple wooden bows. As steel became more widely available in Europe around the 14th century, steel prods came into use.
The crossbow prod is very short compared to ordinary bows, resulting in a short draw length. This leads to a higher draw weight in order to store the same amount of energy. Furthermore, the thick prods are a bit less efficient at releasing energy, but more energy can be stored by a crossbow. Traditionally, the prod was often lashed to the stock with rope,
whipcord, or other strong cording. This cording is called the
The strings for a crossbow are typically made of strong fibres that would not tend to fray.
Whipcord was very common; however linen,
sinew were used as well. In wet conditions, twisted
mulberry root was occasionally used.
Very light crossbows can be drawn by hand, but heavier types need the help of mechanical devices. The simplest version of mechanical cocking device is a hook attached to a belt, drawing the bow by straightening the legs. Other devices are hinged levers, which either pulled or pushed the string into place, cranked rack-and-pinion devices called cranequins
 and multiple cord-and-pulley cranked devices called
Cranequin (rack & pinion)
Cranequin (rack & pinion)
Iron cranequin, South German, late 15th century
A lion holding a crossbow on the coat of arms of
Småland, Sweden. The province's original coat of arms (1560) was of a crossbow alone, the lion being added later.
Crossbows exist in different variants. One way to classify them is the acceleration system, while another is the size and energy, degree of automation or projectiles.
recurve crossbow is a bow that has tips curving away from the archer. The recurve bow's bent limbs have a longer draw length than an equivalent straight-limbed bow, giving more acceleration to the projectile and less hand shock. Recurved limbs also put greater strain on the materials used to make the bow, and they may make more noise with the shot.
Multiple bow systems have a special system of pulling the sinew via several bows (which can be recurve bows). The workings can be compared to a modern compound bow system. The weapon uses several different bows instead of one bow with a tackle system to achieve a higher acceleration of the sinew via the multiplication with each bow's pulling effect.
A compound crossbow is a modern crossbow and is similar to a
compound bow. The limbs are usually much stiffer than those of a recurve crossbow. This limb stiffness makes the compound bow more energy efficient than other bows, but the limbs are too stiff to be drawn comfortably with a string attached directly to them. The compound bow has the string attached to the pulleys, one or both of which has one or more cables attached to the opposite limb. When the string is drawn back, the string causes the pulleys to turn. This causes the pulleys to pull the cables, which in turn causes the limbs to bend and thus store energy. Other types of compound bows use either (one or both) cam shaped or eccentrically mounted pulleys in order to provide a "let off", such that the archer is not holding against the maximum draw weight of the bow while trying to aim. But, in a crossbow, the string is held back mechanically, so there is no advantage in providing a let off. Therefore, compound crossbows generally only use pulleys that are both round and concentrically mounted, in order to capture the maximum available energy from the relatively short draw length.
, a heavy crossbow used for siege defense.
The smallest crossbows are pistol crossbows. Others are simple long stocks with the crossbow mounted on them. These could be shot from under the arm. The next step in development was stocks of the shape that would later be used
for firearms, which allowed better aiming. The
arbalest was a heavy crossbow that required special systems for pulling the sinew via windlasses. For
siege warfare, the size of crossbows was further increased to hurl large projectiles, such as rocks, at fortifications. The required crossbows needed a massive base frame and powerful windlass devices. Such devices include the
torsion springs replacing the elastic prod of the oxybeles, but later also developed into smaller versions.
 Ballista is still the
root word for crossbow in
Romance languages such as Italian (balestra) and Spanish (ballesta).
repeating crossbow automated the separate actions of stringing the bow, placing the projectile and shooting. This way the task can be accomplished with a simple one-handed movement, while keeping the weapon stationary. As a result, it is possible to shoot at a faster rate compared to an unmodified version. The Greek
Polybolos was an ancient repeating ballista reputedly invented by Dionysius of Alexandria in the 3rd century BC. The Chinese repeating crossbow,
Chu Ko Nu, is a handheld crossbow that accomplishes the task with a magazine containing a number of bolts on top. The mechanism is worked by moving a rectangular lever forward and backward. The weapon was mainly used as a weapon against lightly armored soldiers, since it shot small bolts that were often dipped in poison.
A bullet crossbow is a type of handheld crossbow that, instead of arrows or bolts, shoots spherical projectiles made of stone, clay or lead. There are two variants; one has a double string with a pocket for the projectile, and the other has a barrel with a slot for the string.
slurbow is a type of crossbow with a wood or metal barrel over the top of the stock that is arguably influenced by the emergence of the pistol.
Arsenal of ancient mechanical artillery in the
Saalburg, Germany; left: polybolos reconstruction by the German engineer Erwin Schramm (1856-1935)
Chinese Chuangzi Nu stationary windlass device with triple-bow arcuballista
Lian Nu (連弩), multiple shot crossbow without a visible nut or cocking aid
Early modern four-wheeled ballista drawn by armored horses (1552)
16th-century French mounted crossbowman (cranequinier). His crossbow is drawn with a rack-and-pinion cranequin, so it can be used while riding.
Pistol crossbow for home recreational shooting. Made by Frédéric Siber in Morges, early 19th century. On display at Morges military museum.
French cross-bow grenade thrower Arbalète sauterelle type A d'Imphy circa 1915
Arcuballista on wheels with a steel bow and incendiary bolt (15th century)
Modern crossbow bolt compared to a 1
The arrow-like projectiles of a crossbow are called
bolts. These are much shorter than arrows, but can be several times heavier. There is an optimum weight for bolts to achieve maximum kinetic energy, which varies depending on the strength and characteristics of the crossbow, but most could pass through common mail. In ancient times, the bolts of a strong crossbow were usually several times heavier than arrows. Modern bolts are stamped with a proof mark to ensure their consistent weight. Bolts do not have
fletching, i.e. feathered ends like those commonly seen on
 Crossbow bolts can be fitted with a variety of heads, some with sickle-shaped heads to cut rope or rigging; but the most common today is a four-sided point called a
quarrel. A highly specialized type of bolt is employed to collect blubber biopsy samples used in biology research.
Most modern crossbow hunters incorrectly refer to the bolts as arrows, due to the similar appearance, but the physics of how a bolt finds its target are different than that of an arrow used in a vertical bow.
Crossbows can also be adapted to shoot lead bullets or rocks, in which case they are called stone-bows. Primarily used for hunting
wildfowl, these usually have a double string with a pouch between the strings to hold the projectile.
Even relatively small differences in arrow weight can have a considerable impact on its drop and, conversely, its flight trajectory.
The reticle of a modern crossbow
allows the shooter to adjust for different ranges
ancient Chinese crossbow often included a metal (i.e. bronze or steel) grid serving as
iron sights. Modern crossbow sights often use similar technology to modern firearm sights, such as
red dot sights and
telescopic sights. Many crossbow scopes feature multiple
crosshairs to compensate for the significant effects of
gravity over different ranges. In most cases, a newly bought crossbow will need to be sighted for accurate shooting.
Quivers can be mounted to hold ammunition. These are often made from plastic and usually hold the bolts in fixed positions along the structure. A popular detachable design consists of a main arm that is attached to the weapon, a plate on one end that secures four or more individual bolts at a point on their shafts and at the other end a cover that secures their heads. This kind of quiver is attached under the front of the crossbow, parallel to the string and is designed to be quickly detached and reattached. Other designs hold bolts underneath the crossbow parallel to the stock, sometimes on either side of the crossbow.
A major cause of the sound of shooting a crossbow is vibration of various components. Crossbow silencers are multiple components placed on high vibration parts, such as the string and limbs, to dampen vibration and suppress the sound of loosing the bolt.