Ten in a row. The ultimate in one-upmanship in the high-stakes Old Firm rivalry.
The storied accomplishment of 10 successive titles has remained tantalisingly out of touch for both Celtic and Rangers.
Each have faltered in their sole attempt at double-digit dominance. Now Celtic have a second go in 2020-21, having stretched their current streak to nine when last season was ended early.
"The 10" - both winning it and trying to stop it - has become an obsession on opposite sides of the Glasgow divide. But why does it matter so much?
Lisbon Lions blaze a trail
The benchmark was set by Celtic's all-conquering Lisbon Lions. When Jock Stein's men became the first British club to lift the European Cup in 1967, they had already embarked on an unparalleled domestic feat.
From 1966-74, Celtic won nine consecutive top-flight championships and left an indelible mark on the Scottish game.
Alex Miller and Colin Stein were part of the Rangers side who halted their rivals' run, something Steven Gerrard's Ibrox side are hoping to emulate 45 years later.
"The 10 is such a big deal because you're going to make history as the first team to do it. They money you earn from football will go over time, but no one can take away history," says Miller.
Stein, who scored the 1975 title-clinching goal in a 1-1 draw with Hibernian, adds: "There was a lot at stake and it was such a relief, for the team and especially the supporters."
'It's always about trying to outdo Celtic'
The Lions' record stood unmatched for 23 years. The tide began to turn when Graeme Souness' arrival as player-manager at Ibrox ushered in a Rangers revolution of lavish spending.
By summer 1997, the Ibrox club - with Walter Smith at the helm - were one short of 10. This was an absurdly-talented team comprising the likes of Brian Laudrup, Paul Gascoigne, Ally McCoist.
Their momentum looked unstoppable against a Celtic side that had won one trophy in eight years and had just changed manager, appointing Dutchman Wim Jansen. Yet Celtic broke the monopoly, finishing two points clear in May 1998.
That missed opportunity remains a huge source of anguish for former midfielder Charlie Miller.
"The 10 comes down to Rangers and Celtic always wanting to outdo each other," the 44-year-old says. "I'm still sad to this day we didn't win that title. It was a combination of factors - injuries, Laudrup and Walter had both announced they were leaving at the end of the season, Gazza was sold.
"But we should still have won it, we let ourselves down. I wouldn't say the pressure affected us; we more expected to do it, that was the problem. We were so on top during that era."
'Horrendous' pressure & protecting a legacy
In defying the odds to stop the 10, Celtic's players were feted as heroes by their success-starved fans.
Paul Lambert, a midfielder in that side, says the players saw it as their duty to safeguard the Lisbon Lions' legacy. The strain of that became suffocating.
"That pressure was horrendous," says Lambert. "It was the hardest title I won because we were a team just built in a season.
"If Rangers lose 10 in a row now, the history of Rangers is gone. That's the pressure we had. John Clark, Billy McNeill, Bertie Auld, Ronnie Simpson, Jock Stein, Jimmy Johnstone, all those great guys... did you want to be known for losing their history?"
Former team-mate Alan Stubbs agrees: "It was the most intense pressure I've ever played under. It wasn't enjoyable. The rollercoaster of pressure Rangers are going to endure this year will be like nothing they've come across.
"Rangers, when we stopped them, were better than us. There are a lot of similarities to this season."
Lambert believes this season will be a test of colossal test mental strength for Gerrard and his players.
"If you don't stop the 10, you're going to be remembered as a guy who was part of that," he adds. "It's not down to ability, it's in the brain. Mentally can they handle it?"
How do the current players view it?
As soon as Celtic were crowned champions of last season's curtailed Premiership in May, manager Neil Lennon's thoughts turned towards the next campaign.
Its significance is clear to the former captain, who was in charge for the first three titles of Celtic's current streak and has delivered the last two since returning to replace Brendan Rodgers.
"Going for the 10; it's historic, it's momentous and very special," said Lennon. "I know it's been on the tip of a lot of our supporters' tongues and we'll be going all out to do it."
Midfielder Callum McGregor says it will be a "monstrous achievement" should Celtic succeed, adding: "We know what's riding on it this season. It would be something I'd be so proud of."
Rangers are without a major trophy since 2011, and have fallen out of title contention post-winter break in each of Steven Gerrard's two seasons at the helm.
Striker Jermain Defoe, though, is convinced they can harness that disappointment.
"The good thing is that everybody knows how big it is," the former England forward told the Beautiful Game podcast. "I have sensed that in the pre-season, how hard everyone is working.
"We don't want to make the same mistakes we made in the second half of the season. I look at the dressing room and I believe we can go all the way. We just need to believe we can do it this year."
What about the fans?
Celtic supporter Steven Murray, Uddingston, Lanarkshire
The 10 matters so much to me because I grew up in the era when Rangers won nine and were so ruthlessly dominant. The prospect of not only equalling that but going one better is irresistible.
It would also mean so much for Celtic's ongoing upward trajectory and prestige, but the real nub of the matter is, for me, rooted in rivalry.
Rangers fan Andrew Dickson, Motherwell, Lanarkshire
I don't believe 10 in a row means the same as it would have done if Rangers had been in the league throughout Celtic's current run.
It can't be ignored that Rangers were not in the top flight for four years and last season was 'called' prematurely and controversially. This achievement pales in comparison to the previous Rangers and Celtic nine-in-a-row wins.