Surely it is one of those things that you just don't do?

Like sleeping with your wife's sister, not helping an old person across the road or laughing raucously at a funeral service.

But it happens, and it happens with all clubs that absolutely detest a deadly rival. Newcastle and Sunderland, Liverpool and Everton, Celtic and Rangers, West Ham and Millwall - players play for the enemy!

Over the last century, 29 players have worn both the blue shirts of City and the red of United, and lived to tell the tale. Players you've forgotten, players you didn't realise and players who are still around.

It all started with Bob Milarvie, a Scotsman in 1890. With Victorian etiquette at its height, you'd think he'd have known better. However, in 1890-91, Bob played for Manchester United and then transferred to City in 1891 until 1896.

Billy Meredith - City 1894-1906. Billy played 339 times in the blue and again in 1921-24, when he played a further 28 times. For United, he amassed 303 appearances between 1906 and 1921.

Denis Law - The first name that springs to mind when we talk of crossing the great divide. Older fans will remember the cheeky back-heeled goal which was said to relegate United in 1973, but the truth was that United were already gone by then. In fine style, Law didn't celebrate the goal and immediately left the pitch. His halcyon days were in the red and he became part of the trio of players that included Charlton and Best. Law played at City from 1960 to 1961 and then he signed for Torino in Italy before returning to Manchester, but this time in red. 237 goals in 404 appearances from 1962 to 1973, elevated Law to a god-like status and he is still one of their best remembered sons.

Owen Hargreaves - A big money signing from Bayern Munich in 2007, he played 27 times in four years, as injury ravaged his career. After being released by United, Hargreaves took the short walk across town to City, where he made just one appearance - again due to injuries.

Peter Beardsley - Yes, the Geordie was at both clubs. Although he made his name at Newcastle, Beardsley was at United in 1982 for a season, but never played a first team game. By the time he joined City, he was winding down and in 1998 he made just six appearances.

Wyn Davies - Was a well-known player in the early 70's and joined the elite group of players who have played the beautiful game for both clubs. Having joined City from Newcastle in 1971, Davies stayed only one season. In 1972, Frank O'Farrell, the Manchester United manager at the time, took him to Old Trafford for another season, where Davies played just 16 times before joining Blackpool.

Sammy McLlroy - Not many people realise that 70s United icon McLlroy did play for both clubs, but in 1985 when he joined City, he really was at the end of his career, making just 13 appearances. His greatest days were undoubtedly in the red of United and he served the club with distinction from 1971 to 1982, playing 342 times.

Brian Kidd - Kidd is to be seen on the touchline these days, assisting whichever Manchester City manager is in the hotseat. However, it wasn't always that way. Kidd had a long career and was better known at United where he played 203 games between 1963 and 1974. In 1976 he joined Manchester city and racked up 98 appearances.

Andrew Cole - A prolific striker for United after leaving Newcastle. Ferguson signed Cole for £7m in January 1995. He scored 121 times in 275 appearances and was a part of the treble winning United side in 1999. Cole stayed six years at Old Trafford, but in 2005,  just for a season, Cole played just 22 games in the blue of City before injuries took their toll.

Peter Schmeichel - The legendary United keeper played out the final year of his career at the other Manchester club. City won the Manchester derby at home and drew at Old Trafford that season to maintain Schmeichel’s undefeated record in the clash. Another of the treble winning team, Schmeichel is regarded as the best keeper United ever had.

Carlos Tevez - The man who can do no wrong wherever he goes. Still admired at United, still loved at West Ham and longingly remembered at City, where he played with distinction for all three teams. In 2007, Tevez joined United and made 97 appearances, scoring 34 goals and collecting trophy after trophy in a two year spell. In a major coup, City signed Tevez in 2009 and the player went on to win another Premier League title and an FA Cup. In 113 appearances, his goal return was 58 in all competitions for the blue half of Manchester.

That just leaves a surprise for all unknowing Manchester United fans. Records show that Sir Matt Busby was manager from 1945-69 and for the second half of the season 1970/71. Records also show that he played for Manchester City from 1928-1936 making over 200 appearances. Equally, ex-Manchester United heroes Mark Hughes and Steve Coppell also played for one side and managed the other later on. Ernest Mangnall was the only manager ever to manage both United and City in the early 20th Century.

Clearly, you can play for arch rivals and still keep your kneecaps.

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