THEY haven't played each other for three years.
But Glasgow still knows what to expect when Rangers and Celtic meet.
The two teams will be relishing and dreading their cup clash on the field in equal measure.
Whether in blue or green, players know just how much Sunday's League Cup semi-final means to their clubs and their fans.
This fixture, after all, has produced some of the most exciting football anywhere in the world.
It has also provoked scenes - on and off the pitch - that have shamed Scotland.
So Glasgow and much of the rest of the country will share the mixed feelings of players ahead of the game.
Add three years of expectation to Sunday's cocktail, and emotions could not be higher.
After all, fans of one side of the Old Firm were not even sure if they would have a club to support after Rangers - as a corporate entity - were wound up in 2012.
This game is the first time Rangers in its current guise has taken on its greatest rival. Could it be an opportunity for the Old Firm to turn a new leaf?
Many had hoped that the last three years had offered a breathing space for Celtic and the reborn Rangers to rethink what at times has been a toxic relationship between parts of their support. Both clubs had been invited to meet with the Scottish Government's expert group on sectarianism. They have not done so.
The last three years have seen a mushrooming of social media that can amplify the voices of those on the extremes, in our sport as in our politics.
They have also brought growing controversy about whether new laws on offensive behaviour at football are fit for purpose.
Yet something else has happened since Rangers and Celtic last met.
Glasgow and Scotland have shown that we can enjoy a sporting spectacle. Tens of thousands of us watched the Commonwealth Games without so much as a punch being thrown. But the thrill, the thrill was just as great as when one Old Firm team scores against the other.
Think of the buzz that went round Hampden - the same venue as the cup semi on Sunday - when Usain Bolt dashed down the track. Or the joy of Scotland's own boxing postman, Charlie Flynn, saying "you can only kill he crocodile next to you".
The fans who loved the Commonwealth Games? They weren't strange outsiders. They were us. Many of them supporters Celtic or Rangers. Or Aberdeen or Dundee Utd, the New Firm rivals meeting in the other cup semi this weekend. They have set the best example for Hampden.
The police didn't take chances during the Commonwealth Games, deploying more officers than ever before on a peacetime operation. And they won't take chances this weekend. Their feelings will also be mixed.
Police Scotland too is new since the last Old Firm meeting. But officers haven't forgotten how to police such a game. They like the best athletes know a simple truth: practice makes perfect.
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