marking the golf ball on the green

Question
May I mark another player's ball on the green?  Must I ask permission to do so?  Must I replace the ball, or can the player whose ball it is?  There is lots of controversy about this and I would like the correct ruling.  Thank you.

Answer
Penny,

You may mark another player's ball on the putting with that player's permission. The person marking the ball should be the player replacing the ball.

For more in depth decisions, check out the USGA web site under rule 20.

Thanks for the question,

Michael Hoffman
PGA Professional