Even now, when the supporters are increasingly polarised and the antagonism that shapes the Old Firm rivalry has become capable of reaching darker and more violently threatening extremes, there is a sense of Rangers and Celtic being tightly bound together.
The reality of their existence is to provide the means of expression for some old, resentful hostilities, and when the two sides meet again tomorrow, for the seventh and final time this season, the wonder is how the two sides will endure all the tension, the anger, the bitter indignation, and even the angst, of the occasion.
For the away support at Ibrox, much of it will be released in the 18th minute, when they plan to stand and applaud their manager, Neil Lennon. He once wore the No 18 on his back, as a combative midfielder who played with a fierce, almost self-righteous sense of his own authority; now, he represents to the club's fans so many aspects of their identity: Irish background, Catholicism, an unabashed defiance, and persecution.
This first full season in management has seen Lennon occasionally lost to his own ill-discipline, become embroiled in a confrontation between his club and referees and the Scottish Football Association, and be the target for death threats that escalated from bullets in an envelope to a viable parcel bomb.
To Celtic fans, these parcel bombs are treated as proof of a sectarian attitude they believe to still be prevalent in Scotland, and to be perpetuated by Rangers fans. To the followers of the Ibrox side, this is considered an inaccurate slur (since the criminal act of an individual or group cannot be held to be representative of their support as a whole). But then Rangers supporters are currently under investigation by Uefa for allegedly signing sectarian songs in Europa League games last month, and it is undeniable that two offensive anti-Catholic chants, "The Billy Boys" and "No Pope Of Rome", have been heard again this season, having previously been subdued by self-restraint.
When the two teams come together at Ibrox amid this crush of rancour, it will be to contest a game that will have a significant bearing on the title race, even if the game has become obscured by the dramas of this season. It is as if the two teams are united by the need to reclaim their football from the surrounding agitation.
Rangers are one point ahead, although Celtic have a game in hand, yet the players and coaching staff seem reduced by the scale of the turbulence that has surrounded the Old Firm this season. Lennon, in his stark bullishness, has been a central figure, and on the touchline at Ibrox he will be the focus of much of the edginess; this galvanises him, since he is never more alive than when faced with the demand to be emphatic and commanding, but in quieter, more reflective moments, when he thinks of his partner and child also being affected by the strain, then a truer perspective is revealed.
"Anyone in any walk of life shouldn't have to deal with something like this," he says. "It is uncomfortable, you see your face every hour on the news and after a while you start thinking 'is that me they are talking about?' I've had this for 10 years, but I don't want to say you get used to it, because you never do. But it is not going to deter me from doing what I want to do. This is the greatest privilege in my life, to manage this football club, and the support I've had from the fans and my close family and friends has been my strength."
This is Walter Smith's last Old Firm game – in two spells he has managed 55 matches against Celtic, and won 28 – but the only sentiment he feels is a weary sorrow for the kind of spitefulness that prompted someone to send parcel bombs. He understands the need for his side to win the game, but also for the occasion to return to a football rivalry that was once cherished for its intensity, but has now become a source of distress.
"It has been a bad week for the Old Firm historically," Smith says. "In the 20 years I've been involved and being from the west of Scotland, I've never known a week quite like this. It's sad. Regardless of what any of us say, for football to get to this level is entirely wrong. And the people who are behind it ... you struggle to find the words to say how you feel about them."
Ibrox will be the scene of a vital contest, something alive and seething, but that is also capable of becoming engulfed by a looser hatred; one that lacks any kind of restraining perspective.