The Bee Line Putting String may be the most useful training aid of all for putting. The elevated string line serves to train:
building a square setup with good body alignment;
becoming familiar with the look and feel of a line straight away sideways out of the setup;
positioning the eyes and orienting the gaze to see straight down onto the ball so that the string bisects the ball and the sweetspot of the putter head;
monitoring the stroke path back and thru to make sure the putter sweetspot does not get out beyond the line and that in the forward and thru stroke the putter moves straight online and square remaining beneath the string in a square orientation at least for several inches thru impact;
and the proper head swivel that runs the line of sight straight away from the putter face down the string as far as the target, for checking where in fact the putter face aims.
At $12.95, this is about as inexpensive as it gets. Making one on your own entails about $5-10 in materials and the time and trouble of running to the hardware store. Here's the GATW link: Bee Line Putting String.
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