A surprising move with which Tigran Petrosian (1929–1984) met the Queen’s Indian in the 1960s and 1970s is 4.a3. While at first sight this may look like the useless waste of a tempo, it is in reality the ambitious attempt to become active in the centre with Nc3 and e2-e4, without having to fear the annoying pin ...Bb4. It was not until Garry Kasparov won a lot of games with it in the 1980s that the Petrosian System really became particularly popular. For Black the most important replies are the traditional move 4...Bb7 and the really unusual looking extended fianchetto 4...Ba6.
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