A vang is a sailing term referring to a piece of rigging used on sailboats. Specifically, it refers to the boom vang, which controls the angle of the boom on a sailboat. The boom vang is used to:

Tension the sail, flattening or loosening it depending on the wind conditions.
Control the vertical movement of the boom, which helps in shaping the sail to optimize performance.
The boom vang can be a mechanical device with blocks and lines or a hydraulic system, depending on the size and type of the sailboat.

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