Germany's Matti Schmid used his broom-handle putter to address his ball as he faced another crucial tap-in en route to second place at the Charles Schwab Challenge on Sunday - his best PGA Tour finish.

Briefly, the television cameras showed the top end of his club. A clenched left hand had its thumb resting on top of the grip. His blue patterned shirt creased around the knuckle of his thumb.

The image begged some questions: was that knuckle also in contact with his chest? If so, was it wedged against his sternum? Had he found a fixed anchor point? Was he breaking the rules? Or was it OK?

BBC Sport has contacted Schmid's representatives, but the very fact his technique could be questioned - with no definitive answer - means a brilliant putting display could fall under an unwelcome spotlight.

Rule 10.1 of the Rules of Golf states: "When making a stroke, a player must not anchor the club" by "doing it 'directly', by holding the club or a 'gripping hand' against any part of the body".

There was certainly no daylight between Schmid's left thumb and his shirt, and when I showed a screengrab of the image to a leading referee he acknowledged it was a "bad look".

But he also pointed out that the rules state: "If the player's club, gripping hand or forearm merely touches their body or clothing during the stroke, without being held against the body, there is no breach of this rule."

So we - as observers - do not know whether Schmid's putt was legal or not.

Despite legislation coming into effect almost a decade ago, the issue of 'anchoring' continues to be one of the murkiest in the game.

If anything, it feels as though it is becoming ever more pertinent.

Schmid topped the putting stats at Colonial as he finished second to Ben Griffin. After gaining more than eight strokes on the field with his prowess on the greens, that runner-up finish propelled the two-time former European Amateur champion to 55th in the PGA Tour's FedEx Cup standings.

None of this is to say the 27-year-old's action broke any rules. There may be enough lateral movement in his stroke to suggest he is not stabilising to a fixed point.

But the broader topic remains one of the hottest in professional locker rooms.

"He's probably OK," the referee I spoke to added. "He's probably just touching the shirt material, but by what he is trying to achieve he's going to be as close as possible to his chest, and therefore player integrity comes into it."

Another official, who referees on the DP World Tour, said on condition of anonymity: "It is super difficult to enforce.

"We have spoken to a couple of players who we felt were getting close to breaching the rule and they actually have gone back to shorter putters, which is good.

"But ultimately you have to go with the integrity of the players. How do you enforce it? It is your word against theirs. You're in no man's land."

The rule was introduced in 2016 after a spate of majors were won by golfers using extended putter shafts.

Keegan Bradley anchored in his midriff to win the 2011 US PGA, as did Ernie Els for his Open triumph the following year. There were broom-handle major wins for Webb Simpson in the 2012 US Open and Adam Scott a year later at the Masters. The Aussie continues to use the long putter, but with daylight between his stroke and his body.

Protecting the "fundamental characteristics of the putting stroke" was the reason for banning anchoring nine years ago.

It is OK for an extended shaft to be clamped against a forearm (as US Open champion Bryson DeChambeau does) because that moves to make the hit.

But there are those in the game who feel even this technique makes putting "too easy".

And there are plenty more exponents who continue to shun convention and go with longer implements. Along with Schmid, the likes of Akshay Bhatia, Si Woo Kim and Lucas Glover use these potentially contentious clubs to great effect.

Bernhard Langer and Scott McCarron have long endured their techniques being questioned on the Seniors tour, and Spaniard Eugenio Chacarra's Indian Open win on the DP World Tour last March drew focus.

Langer and McCarron have vehemently denied breaching the rules.

Back in 2017, Langer said: "I personally don't understand it because I'm a man of integrity, and the last thing I want to do is break rules and be known as cheating.

"I know when I'm anchoring and when I'm not anchoring because I control my hand. It goes sideways, you can see it.

"So when my knuckle is away from the body, I'm not anchoring or touching any part of my body and I know I'm within the rules."

The problem is that in many cases it is not evident to the naked eye and is impossible to enforce either way.

It means an issue as fundamental as the putting stroke is shrouded in more grey areas than a flotilla of battleships, which is highly unsatisfactory and not fair on the likes of Schmid when they dominate so brilliantly on the greens.

DP World Tour winner Eddie Pepperell believes the solution is "quite simple".

"Insist on the putter being the shortest club in the bag," he told the Chipping Forecast podcast.

Some referees agree.

"Easy solution is just limit the maximum length of the putter and be done with it," said one official.

Another said: "The powers that be should be looking at it. If there is a rule, we should be able to enforce it."