A riding sail (or steadying sail) is a small, typically triangular sail set at the stern (often on the backstay) of a vessel while at anchor or moored. It acts as a stabilizer to reduce yawing (side-to-side swinging), keeping the bow into the wind to increase comfort and reduce anchor load.

Key Aspects of a Riding Sail:
Purpose: Minimizes "sailing" at anchor, reducing strain on the anchor gear and improving stability.

Operation: It is hoisted aft and sheeted flat to create wind resistance, preventing the boat from sheering back and forth.

Usage: Effective in high wind conditions, particularly for boats with high profiles or shallow drafts that tend to swing.

Alternative Uses: Can be used to keep a boat steady when using a sea anchor.

Often, it is a specialized storm jib or a dedicated triangular sail designed for this specific purpose.