A topping lift is a line (running rigging) that supports the aft end of the boom, holding it up when the mainsail is lowered, reefed, or while docked. It runs from the masthead to the boom end, preventing the boom from falling into the cockpit or onto the deck.

Key aspects of the topping lift:
Purpose: Keeps the boom off the deck, bimini, or crew, and allows for adjusting boom height when the sail is down.

Operation: It must be loosened or slackened once the sail is raised; otherwise, it will cut into the sail, creating a "hook" and ruining sail shape.

Alternative Uses: It can serve as a spare main halyard in emergencies or be used to adjust boom height to ease reefing.

Similar Systems: On many modern boats, lazy jacks or a solid boom vang have replaced the need for a traditional topping lift.

Note: The topping lift is often confused with a "quarter-lift," which is an older, alternative line used on some boats to manage the boom.