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Trim tabs are small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft. The angle of the tab relative to the larger surface can be adjusted to null out hydro- or aero-dynamic forces and stabilize the boat or aircraft in a particular desired attitude without the need for constant control inputs.

When mounted on a movable surface (such as a rudder), the trim tab acts as an anti-servo tab, moving in opposition to the main surface. Because the center of effort of the trim tab is further away from the leading edge of the main surface than the main center of effort, the forces generated by the tab work over a longer lever arm. The main surface will rotate around its mounting axis until the moments from the main and trim surfaces balance each other.

High-speed large powerboats (30ft+) and large cruisers (30ft+) with planing-type hulls will often have trim tabs attached to the trailing edge of the hull or transom. These are used to adjust the pitch attitude of the boat while underway. Changes in boatspeed will usually require the trim tabs to be adjusted to keep the boat at a comfortable and efficient pitch attitude. http://www.trim--tab.com

In some sailboats, the trailing edge of the keel has a trimtab which is used to null out rudder forces (lee or weather helm).

Sailboats intended for long-distance voyages with minimum crew often have a self-steering system which works via a trim tab (usually called an anti-servo tab) on the trailing edge of the rudder. This tab is mechanically connected to a windvane. The system automatically adjusts the boat's heading to keep it at a fixed angle of attack to the apparent wind.

Virtually all airplanes (including gliders) have elevator trim tabs. Elevator trim frees the pilot from constantly adjusting the pitch controls. A pitch trim wheel is adjusted by the pilot to cancel out control forces for a given airspeed. When the trim wheel is rolled forward the nose is held down.

Many airplanes also have rudder and/or aileron trim systems.

When a trim tab is employed, it is moved into the windstream opposite of the control surface's intended deflection. For example, in order to trim an elevator to hold the nose down, the elevator's trim tab will actually rise up into the windstream. The increased pressure on top of the elevator surface caused by the raised tab will then deflect the entire elevator slab down slightly, causing the aircraft's nose to move down.