[O.B.Left]

The string line is the inclined plane too and shares a similar geometry to your plane board or a putting arc. Arc of Approach, not straight back straight through. Think of it as a horizontal line on the face of your plane board that you run the shaft along. You could imagine a number of strings laid out on top of each other and forming the inclined plane. Very different geometry than the a straight rail you run the clubhead along or two straight rails like the old Pelz putting rail. Although there's nothing wrong with that for putting according to Homer, just dont do it for full swings. Dont Cover the Plane Line that is.

The advantage of the string line is that you are not resting the putter head against anything. Its free flowing and can sag off plane if you're not careful. You are also not tempted to Cover the Arc of Approach with the clubhead, a questionable habit. Far better to trace the straight line plane line in my opinion and observe the visual equivalent of the Arc of Approach method's slightly arced clubhead blur.

Me I hardly watch the clubhead at all , or the ball.