A tractor beam is an attenuated linear graviton beam used by starships and space stations to control the movement of external objects. The beam places spatial stresses on the object in specific areas allowing the ship using the tractor beam to hold the object in a fixed location or alter its position or trajectory.
Tractor beams are generally only used at sub-warp velocities. Using a tractor beam at warp speed is extremely dangerous (and therefore rarely performed) due to the increased stresses between the vessels, which can severely rupture the hulls of both vessels. Using a tractor beam can be done at warp speed only if both vessels' speeds are exactly matched. If able to engage its tractor beam on another vessel at warp, it will usually have to disengage the tractor beam before either vessel can return to sub-warp velocities. However, if both vessels' hulls are strong enough to withstand the stress, they can both be brought out of warp with the tractor beam still engaged if the towing vessel carefully disengages the warp drive while engaging the impulse engines.
Most Federation vessels are equipped with a tractor beam emitter on the aft ventral hull, as it is the best location for towing objects. Secondary emitters are also placed in other locations. Smaller tractor beams are installed within shuttlebays to assist in docking and landing maneuvers.
A carefully modulated tractor beam can be used to affect the effectiveness of ship-to-ship weaponry. In 2372, Lieutenant Commander Worf of the USS Defiant suggested using such a beam against a Vor'cha class attack cruiser. When put into practice his plan was successful, deflecting some of the vessel's disruptor fire and reducing its effectiveness by 50%.