Leading edge extensions or LEX (also referred to as leading edge root extensions or LERX or strakes or chines) are fillets added to the front of a modern fighter aircraft's wings in order to provide usable airflow at high angles of attack. They are typically roughly triangular in shape, running from the leading edge of the wing root to a point near the cockpit along the fuselage. They tend to be fairly small in span, extending out less than a metre. In effect, they are small delta wings grafted onto the front of the normal wings.
In cruising flight the effect of the LEX is minimal. However: when the angle of attack increases, as in a dog fight, the LEX starts to generate a high-speed vortex that remains attached to the top of the wing. Due to the effects described by Bernoulli's principle the wing therefore has a low pressure zone on top, and continues to generate lift past the normal stall point. The F/A-18 Hornet has especially large examples of LEX, as does the Sukhoi Su-27. Early prototypes of the Su-27 crashed due to poorly designed LEX, causing it to freeze at angles of attack above 5 degrees. This has since been overcome. In fact, the LEX help in making advanced maneuvers such as the Pugachev's Cobra, the Cobra Turn and the Kulbit possible.