Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. d4/4...exd4/5. O-O

= Two Knights Defence: Classical Variation =

5. O-O
The classical variation. Castling both gives the White King some security and activates the rook to be able to come to e1 which has much potential to support the White e-pawn or pin any pieces that take the e-pawn. Black's possible sensible replies are:
 * 5....Nxe4
 * 5....Bc5
 * 5....d6
 * 5....Be7

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Theory table
.
 * 1. e4 e5 2. Nf3 Nc6 3. Bc4 Nf6 4.d4 exd4 5. O-O