A player in the group chipped up onto the green today, and his ball came to rest in the line of another player. My ball was in a similar position, and I had already marked it. So I wanted to mark his ball so that the other player could putt while the first player retrieved his putter.
First, you need another player's permission to mark their ball (unless you're their caddie). And you can't just declare that it's always okay. You have to give permission each time. This was a casual round with players that I often play with, so in the spirit of the game, and for pace of play, I was going to mark his ball, but I didn't have a second ball-marker.
I marked it with a leaf, which I was pretty sure you're not allowed to do. The definition of a ball-marker indicates that it is an artificial object.
Why? Because a leaf may blow away? Some random artificial object could blow away as well. Because there could be confusion over which leaf was marking the ball? Maybe....
This is all covered by Rule #14. It's a one-stroke penalty.
So who gets the penalty here? I guess I do, for marking his ball
without explicit permission, and for using an illegal natural
ball-marker.
I found it interesting that the rule explicitly indicates that it's okay to mark your ball with a club right next to the ball. I'm pretty sure that's a recent addition.