The pendulum stroke is considered by many to be
the most reliable way to execute a putt.
The Putting Triangle formed by your shoulders, arms, wrists, and hands is the basis of a pendulum stroke. The pivot point for this stroke is at the base of your neck.
The upper body arrangement is commonly referred to as the Putting Triangle.
The reason for moving the triangle during your putting stroke is that it takes your wrists out of play, allowing your shoulders, arms, wrists, and hands to function as a single unit.
This is the technique most favoured by Tour Players. Certainly it is the way to putt most consistently and when you are in a pressure situation.
According to the dictionary a pendulum is a weight suspended on a rod, string or wire attached to a fixed pivot point. It swings freely from this point under the influence of gravity in a back-and-forth motion.
Strictly speaking, therefore, the shoulder-arm movement of your putter is not a true pendulum as there is no fixed pivot point to guide your swing.
As Geoff Magnum of PuttingZone explains there isn't a single 'arm' suspended from a single pivot as with a pendulum. Instead, you have more of a three-piece triangle connected to your shoulder frame, and your shoulder frame rocks back and through by virtue of torso rotation.
Bobby Jones writing in The American Golfer in March 1930 was dismissive of the pendulum stroke in putting, claiming that it was a physical impossibility in actual application.
The anchorage or pivot point of his putting stroke was his right forearm. This was because earlier golfers putted with their wrists and kept their shoulder frame the same way as it was at address.
Most golfers putt with a short putter. However, it is important to appreciate that your putter shaft is not part of the pendulum. The pivot of the stroke is not in your wrists, but at the base of your neck.
Some people believe that the belly putter has an advantage over the short putter because there is a fixed pivot point on your stomach. In the long putter it is under your chin. Unfortunately this is not the case.
(From 2016 anchoring the putter to your body is banned. The putter can still be used, but the grip must be away from your body)
The advantage of the belly putter / long putter is that it helps you to release the putter head through the contact area without breaking your wrists.
The pivot point is still at the base of your neck unless you are only using your arms with limited shoulder movement to execute your putt.
The closest approximation to the pendulum is the long putter when it is positioned under your chin. (The change of rules means that the putter must not be in contact with your upper torso). The putter shaft forms the rod with the putter head the weight at the end.
However, because all putter shafts must diverge from the vertical by at least 10 degrees, the putter will swing on a slight angle rather than vertically under the pivot point.
All of this is somewhat academic. The more you can model your putting stroke on the steady rhythm and tempo of a weight swinging from a fixed pivot point, the better you will putt.
Because there is no fixed pivot point in real life, it does not mean that you cannot simulate a pendulum-like stroke.
1 = Image from EGO Putting Coach Training Aid
2 = www.life.com