Actually, it was Best of 16 or first to win 4 games, whichever came first.

<Hesam7> what was the reason given? petrosian tried to get the match overturned...what could have been his reason for this i wonder?

Here's the way Jude Acers tells the story at

http://www.chessdryad.com/articles/...

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Korchnoi played beautifully to achieve three wins, one loss, and one draw in the first five games against Tigran Petrosian in Odessa, April 12-24, 1974. Just after the fourth game Tigran Petrosian went to the match committee and requested in writing that Victor Korchnoi be asked not to move his leg up and down beneath the table so much! It was just a Korchnoi nervous habit and did not seem to disturb anything really. No noise or offence intended probably. But Petrosian mentioned that Korchnoi had actually kicked him beneath the table while reaching out to make a move. Surely it was an accident....

Korchnoi knew absolutely nothing of Petrosian's complaint throughout the night, and it was only upon arriving for the fifth game that he was shocked by the formal request to quit moving his leg in a kicking motion beneath the table! Korchnoi was furious but did not say anything to his opponent, beginning to make moves against Petrosian in the fifth game.

You're not going to believe what happened next and at the worst possible moment. Petrosian, while shifting in the chair to adjust his hearing aid, kicked Victor Korchnoi accidentally! As match officials looked on with complete horror and silence. Everybody knew the match could explode any second.

Korchnoi, now thoroughly in flames, sat there for a second and found what has to be one of the truly great one-liner punch outs of all times... "Mister Petrosian, please look for your match chances above the chess table rather than below it." That's the real story, how a great match really ended -- never reported by the wire services.

Petrosian exploded, refused to continue the fifth game, and resigned the match forthwith.

Korchnoi vs Petrosian, 1974

As for the other two instances, I'll pose that as a quiz question to our readers....