A small, flat object — a coin, a tee, or a purpose-made disc — placed right behind or right beside the ball before it's lifted, so it can be replaced on the exact spot afterward under Rule 14.1. Marking is required whenever a Rule calls for the ball to be replaced, such as lifting to clean, identify, or clear another player's line on the green; it isn't required when lifting to take relief. There's no penalty for using a marker itself, but lifting without marking, marking incorrectly, or making a stroke with the marker still in place each cost one stroke. A marker sitting in someone else's line is shifted a clubhead or more to the side using a second reference point, then returned before the putt.