THE argument has been running longer, many years longer, than Celtic's clean-sheet record in the SPFL Premiership.

And, after the Hoops saw their domestic defensive blank sequence come to a shuddering halt against Aberdeen at the weekend, it is once again the subject of passionate debate in stands, pubs and living rooms around the country.

Why do managers, including Neil Lennon and Gordon Strachan, favour zonal marking when defending set-pieces?

Detractors will claim deploying this set-up allowed Russell Anderson to pounce on a loose ball and crash home Aberdeen's equaliser to end Fraser Forster's dream of setting a new standard for shut-outs.

The layman's view is that when the attacking team have control of the delivery of the ball, they can choose where it is going to go and simply overload that area of the box, hence increasing greatly the chances of their side getting to the cross and having a shot or header at goal.

The advocates of the system - and there are many - counter that, if everyone is doing their job correctly and diligently, the defending team has every area of the box covered and the opportunity for an attacking player to find the space to get to the ball is minimised.

Famously, Strachan and Graeme Souness got into an animated on-air argument about the merits of zonal against man-marking as they sat in the Sky Sports studios picking the bones out of the 1-1 Champions League draw between Arsenal and Liverpool.

The Gunners had taken full advantage of the Reds players holding their allotted positions as Emmanuel Adebayor stole in and scored their goal - and it infuriated the former Anfield skipper and manager.

As the credits rolled at the end of the programme, Strachan - who said he had used zonal marking while taking Celtic to the last 16 of the Champions League for the first time in their history - was still trying to convince his former Scotland team-mate of the merits of the system.

Six years on, and the debate still rages, fuelled further by Saturday's events at Parkhead.

Strachan continues to be a fan of zonal, but explains there are golden rules which must be followed if it is to be effective.

He said: "The thing about zonal marking is you have to keep your nerve and not move until the ball is kicked."

Lennon is certainly keeping his nerve, and is stout in his defence of a tactic which brings so much criticism whenever a goal is conceded.

He believes it is disproportionate to the flak which flies if a team deploying man-marking concedes at a set-piece.

Lennon points out that, for the vast majority of time, zonal marking has allowed Celtic to keep the opposition out.

So, when asked to defend its use in the wake of his side losing their first goal in domestic competition since November 23, he fired from the lip.

"It has nothing to do with zonal marking," he insisted.

"People go on about it. But, every time a goal is conceded when a team is man-to-man marking, there is no big debate.

"We have been zonal marking for the past three years, and I think when we played AC Milan here in November was the first time we conceded direct from a corner in 18 months.

"On Saturday, it was people not taking their responsibilities in the position they are supposed to be in."

The one area in which everyone can agree is that, whether it be man-marking or zonal marking, if a player does not attack the ball when required, you are left exposed and vulnerable.

Celtic's favoured formation is to put a player on each post and four along the six-yard line. On Saturday, Anthony Stokes was at the front, with Efe Ambrose, Virgil van Dijk and Georgios Samaras lined up at equal intervals behind.

Barry Robson's delivery for the equaliser saw the ball drop between Van Dijk and Samaras. It came off Andrew Considine's chest. Anderson spun, after unsuccessfully challenging for the ball with Ambrose as it flew over their heads, and volleyed home before Stokes could block.

Lennon knows the goal could have been prevented. But, having studied how Aberdeen try to exploit set-pieces, he believes every precaution had been taken in the way his men set up.

"We do half and half," he said. "We are zonal across the six yard box, and we are man-to-man everywhere else.

"On Saturday, the ball was there for either of my centre-halves to go and head, but we didn't deal with it. Someone has to attack the ball."

It was not the only time the defence - so lauded and assured before last weekend - was struck by torpor.

The winning goal, from a long free-kick down the centre which Ambrose then Stefan Johansen failed to effectively clear, was eminently avoidable.

Aberdeen had chances to extend their lead, though, in mitigation, as the clock ran down, Celtic committed more and more men forward.

However, Lennon is trying to understand why a defensive unit which had been the foundation for Celtic's power surge towards a third title could suddenly look so out of kilter. He said: "I don't know where we went on Saturday defensively because we had been so assured.

"Even the two full-backs were not at their best, for some reason. I can understand Adam Matthews, to be fair, as he was coming back from an injury. But there was no spark about Emilio Izaguirre's performance, either."

Lennon is sure there will be a positive reaction when they run out again, against St Johnstone on Sunday.

Their clean-sheet bubble has been burst, but pride has also been pricked, and the manager warned: "The criticism will come, and they have to take it."

Perhaps the last word on the zonal marking/man-marking argument should go to Lisbon Lion, Tommy Gemmell.

While he famously scored in two European Cup finals, he also knew a thing or two about defending.

Gemmell believes zonal marking would have been anathema to the greatest Celtic manager of all time, Jock Stein.

Big Tam said: "I would love to have seen Big Jock's face if we had lost a goal and a player said 'But I was guarding the space, boss'."