The face of your putter can still rotate open and closed even if you keep your stroke on plane. The Putter Guide adds a flat edge to the back of your putter making it easy to see, using any rail type putting board, if your putter is truly rotating open or closed. If the Putter Guide is flush against the rail then the face of your putter is square to the plane of the stroke. This will make the putter face square to the ball and allow you to hit your putts solid.
The Putter Guide easily attaches to any mallet or blade putter.