In 2018 and before:

  1. If the ball was moved by any player, caddie or outside agency (such as an animal, spectator or moving object) it must be replaced on its original spot.
  2. If however the ball was moved by wind, water or other natural forces (including when the ball moved for no apparent reason; gravity say) it must be played from its new spot.

In 2019, the second situation (ii) changes and will depend on whether or not the ball had been lifted and replaced on its original spot before it moved:

  1. If it had been lifted then the ball must be replaced on its original spot, regardless of what caused it to move.
  2. If not, it must be played from its new spot.

The Ruling Bodies give these reasons for the change:

  • When a ball at rest is moved by natural forces such as the wind, it is normally played as it lies because its movement is considered a continuation of the previous stroke, as no person or object has affected where the ball lies.But when the moved ball had already been lifted and replaced, the connection to the previous stroke has been broken. Notably on the putting green, where a player is allowed to mark, lift and replace a ball for any reason and many players do so as a matter of course.
  • It can be difficult to determine whether a person or object caused the ball to move or whether it was moved by wind or other natural causes. It will simplify the Rules to provide that a ball always must be replaced if it moves from a spot where it had already been lifted and replaced.
  • When a ball on the green moves after having come to rest: It can result in outcomes that seem unfair, such as when the ball rolls off the green (sometimes ending up in a bunker or in water) or rolls close to or into the hole,  Requiring the ball to be replaced if it had already been lifted and replaced will eliminate such outcomes in those situations.
  • This Rule change may also be helpful in conditions of very high wind on the course, as it may allow play to continue in conditions where it might otherwise not be possible or fair because too many balls are being blown from their spot on the green.

In addition, it is arguable that the player may take the opportunity delay replacing the ball for a few seconds to allow the wind to abate.