Can you put your putter head on the line of your putt that you are about to take?

[SwingsitlikeHogan]

There is no specific constraint in the rules.
It simply says - An artificial object when used to mark the spot of a ball to be lifted, such as a tee, a coin, an object made to be a ball-marker or another small piece of equipment.
However:
A player may use equipment to help his or her play during a round, except that a player must not create a potential advantage by:

• Using equipment (other than a club or a ball) that artificially eliminates or reduces the need for a skill or judgement that is essential to the challenge of the game,

How do players using DMDs avoid falling foul of this - which they clearly do.

 

[rulefan]

Because Rule 4.3a(1) specifically permits it.

 

[chrisd]

Also, distance is a matter of "fact" and therefore " skill or judgement that is essential to the challenge of the game" is not affected - a player, back in the day had yardage books, on course distance markers and often paced out yardages to work out the distances. dmd's have just speeded up the process