411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nf6/3. Nc3/3...Ng4
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Petrov's Defence, Vladimir Variation
This is the Vladimir Variation of the Petrov's Defence. It is considered dubious for these reasons: It wastes tempo by moving the same knight again which is considered time-consuming. It also moves the knight to an awkward square, so it can easily be attacked. Finally, the knight on g4 isn't that effective and it is in danger of getting trapped.
1. e4 e5 2. Nf3 Nf6 3. Nc3 Ng4