It was developed from the post-classical Roman onager (wild ass), which derived its name from the kicking action of the machine. The onager consisted of a frame placed on the ground to which a vertical frame of solid timber was rigidly fixed at its front end; through the vertical frame ran an axle, which had a single stout spoke. On the extremity of the spoke was a cup to receive the projectile. In action the spoke was forced down, against the tension of twisted ropes or other springs, by a windlass, and then suddenly released. The spoke thus kicked the crosspiece of the vertical frame, and the projectile at its extreme end was shot forward.
Some authorities think that the trebuchet may have developed from the stave sling used widely by ancient and medieval armies.
A trebuchet is moved by a counterweight. The axle of the arm is near the top of a high strutted vertical frame. The shorter arm of the balance carries the counterweight and the longer arm the sling that carries the shot. The sling is braided from rope, and has a captive end attached to the arm, and a free end whose loop slips from a hook. A trigger, usually a toggle in a chain, holds the arm down after the trubuchet is cocked. Cocking is often performed with windlasses.
In operation the long, nonweighted end is pulled toward the ground, and held by a trigger. When the trigger is released, the arm pulls the sling out of a channel in the base of the frame. When the ball is moving at 45 degrees upward, the free end of the sling slips from the hook, and the missile flys free. The trebuchet's arm and frame then oscillate for several cycles.
The efficiency of a trebuchet can be improved by helping the weight to fall more nearly straight down. One cheap method is to place the weight in a swinging or jointed bucket. The sand or stones in the bucket can also be less expensive than fixed metal weights. Another trick is to place the supporting frame on wheels. Either of these improvements can add thirty percent to the throwing distance.
Aiming a trebuchet is best practiced with a scale model. Usually small adjustments in elevation can be made by changing the angle of the hook holding the free end of the sling, a process which requires a heated forge on a full-scale engine. The perfect release angle is when the missile will fly at 45 degrees, because this optimizes range. After maximum range is achieved, the trebuchet can be moved toward or away from the target. Small adjustments from side-to-side can be made by moving the channel in which the missile and sling slide in the base of the frame.
Trebuchets were first used in Italy at the end of the 12th century, and were introduced in England in 1216 during the Siege of Dover.
Trebuchets were capable of reducing cities to rubble, for they were able to strike from far away, where arrows could not reach their operators. The trebuchets were also able to throw large stones, cows or even shunned negotiators. Rotting flesh (to introduce disease) and barrels of burning tar or oil (to burn the defenders) were also popular.
A simulation of trebuchets in action can be seen in the 2003 movie The_Lord_of_the_Rings:_The_Return_of_the_King where they are used in city defense.