Somewhere along the way, you probably heard that it's okay—maybe even to your advantage—to swing the putter straight back and straight through. But a simple exercise you can do at home will make it clear that the best path takes the putterhead slightly inside the target line on the way back and then back inside after impact. This arc-like stroke is the one used by almost all good putters. Why? Because it makes it easier to strike the ball squarely and with the correct amount of loft.
THE TEST: PUTT ALONG A 2 X 4
Find a flat putting surface and set a 2 x 4 on the ground (you can also use the baseboard along any wall at home). Place a ball on the ground just inside the 2 x 4 and sole your putter behind the ball so that the toe just barely contacts the wood. Now try to swing the putterhead straight back while maintaining constant contact with the 2 x 4. It doesn't feel natural, right? That's because to take the putter back dead straight, your right elbow has to separate from your body, like a chicken wing. This delofts the putterface and makes it difficult to contact the ball on the center of the face and with the correct amount of loft.
THE SOLUTION: SWING ON AN ARC
This time, start your stroke with the putterhead touching the board. But as you swing the putter back, let it naturally lose contact with the 2 x 4 and arc to the inside. As you return to impact, the toe of the putterhead should just brush the board and then again arc to the inside and away from the board, as it did in your backswing. As you take the putter back, don't rotate your hands—this opens the face. Instead, simply rock and turn your shoulders to create the arc.