One should think about the body parts – shoulders, hips, knees, and feet –being aligned parallel to the aim line. A way to think about this is, as if your toes were on one rail of a railroad track which is pointing along the target line and the putter on the other rail.
This alignment is important because it enables the body to swing naturally with the shoulders, arms, wrists, hands, and the putter acting as a single unit — pivoting about the spine in a plane moving toward the target line.
If the body is not positioned parallel to the target line, and thus wants to swing in a different direction, one has to manipulate the stroke to compensate for the miss-alignment.
This adds another source of error which leads to inconsistency.
Need extra help with your alignment? Be sure to read The Fundamentals of Putting and learn more about our Frankly Frog Putters with Alignment Guidance System.