What provides directional (yaw) stability for an aircraft?

Directional stability is primarily provided by the vertical tail (fin). When the aircraft yaws to create a sideslip angle, the vertical tail experiences an angle of attack that generates a side force, creating a restoring yawing moment to reduce sideslip. The effectiveness depends on vertical tail area, aspect ratio, and moment arm from CG. Additional contributions come from fuselage shape and wing sweep (swept wings contribute to directional stability). Adequate directional stability is essential for coordinated flight and spin resistance.