Captains Allister Hogg and Ali Kellock will put the brakes on their friendship for 80 minutes when Edinburgh and Glasgow go head to head after Christmas in the Magners League.
Adding extra spice to the Murrayfield occasion will be the incentive for the winners of clasping one hand on the 1872 Challenge Cup, which dates back to the occasion the teams from the cities first met.
Glasgow travel to Edinburgh for the clash on December 28 which will determine which of Scotland's two professional teams end the year the highest in the league table.
Just one point separates the sides, with Glasgow standing fifth and Edinburgh in sixth after eight matches.
And although Hogg and Kellock have been friends since childhood, the Scotland internationals will be fierce rivals when their respective teams go head to head.
Edinburgh back-row forward Hogg said: "I think there will be a laugh and a joke perhaps at the coin toss, but once you're out on the pitch it is totally different; you just want to perform to your best and win.
"Ali and I played for Stirling together and then when we played for Edinburgh together we used to share lifts to and from training and matches.
"So I know him really well and he is a good friend. I've only ever played against him once before and it is always good fun to play against your mates because it adds an edge to the game."
The 1872 Challenge Cup will be awarded to the aggregate winners of the two matches between the teams this season. They meet again at Firhill in April. Edinburgh are the holders and will retain the trophy in the event of a tie.
Glasgow skipper Kellock returns to Edinburgh to face his former club, and said: "There is rivalry between the two teams because of the history of the fixture, but there is also a rivalry between the cities on the whole as far as any sport is concerned.
"I've known 'Hoggy' for about eight years and we are good friends. It is a great coincidence that he is now captain of Edinburgh and I'm captain of Glasgow and it adds a wee bit to the game."
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