I know many players use a line on their ball to show the starting line for a putt. I believe this violates the Rules of Golf for the following two reasons:
1. I believe having a line on the ball violates Rule 8-2b. The line on the ball is present when the stroke is taken and, by definition, it's a mark.
From the Rules of Golf:
"8-2. Indicating Line of Play
b. On the Putting Green
When the player's ball is on the putting green, the line of putt may be indicated before, but not during, the stroke by the player, his partner or either of their caddies; in doing so the putting green must not be touched. A mark must not be placed anywhere for the purpose of indicating a line of putt."
2. Lining up putts this way definitely slows down the game. How many times have we seen pros mark the ball, adjust the line on the ball, step back to take a look and then re-mark the ball so they can re-align? That's a common occurrence and it's often imitated by amateurs. I don't know whether this technique actually results in more putts made which, I suppose, could speed up play, but I doubt it, especially at the amateur level.
Is there another side to this issue that I'm not seeing? I look forward to other, constructive viewpoints.