What makes a footballing rivalry?
A Dundee United-supporting mate of mine sends Dundee Football Club a remembrance card every September, to remind them of a famous date for United fans of a certain vintage.
The card celebrates United's 5-0 win at Dens Park against Dundee FC, on 11 September 1965, a game referred to in song by United fans, as the "Dens Park Massacre of 65".
For obvious reasons I won't divulge his identity.
But, does his faithful posting of the card every year make him a sad Tangerine basket case, or should we see his behaviour as a celebration of the kind of humorous and harmless rivalry which football fans often indulge in?
Rangers' Maurice Edu and Celtic's Beram Kayal battle for possession during one of last season's Old Firm matches Pic: SNS
Billy is a great and knowledgeable football man, but what kind of reception will he get from the Easter Road faithful after his years of service to their rivals Hearts.
Hopefully it will be a reception based on the strengths which he will undoubtedly add to their backroom staff.
But with football fans you never know.
Just what is it that makes football rivalry so intense among fans?
In some cases, such as the Old Firm, there are obvious historical differences between the two sets of supporters.
In other cases though it is much harder to explain the rivalry which often sees otherwise intelligent human beings show an almost pathological dislike of their rivals.
So what is it that divides otherwise sane and rational men and women when it comes to football and should we laud it or loathe it ?
From the Old Firm, to the Great Divide's of Dundee and Edinburgh; from the rivalry of Raith Rovers and Dunfermline, to the simmering antipathy between St Mirren and Morton, and right down even into the junior ranks, what is that supporting a football club does to affect the normal behaviour of otherwise rational folk?
Answers on a postcard please.