The Dave Pelz Putting Tutor has two marbles at its front edge, which tells you whether or not you’ve made a “pure strike” and started a putt on line. If your putt dislodges a marble, it means your ball started off-line. There are 3 different marble gate positions standard, pro and super pro.
Click here for more info: Pelz Golf Putting Tutor

Dave Pelz Putting Tutor