Here is a quick trap in the opening which players need to be aware of. It arises in the Petroff's Defense after these moves –

1 e4

The favorite move of American World Champions from Paul Morphy to Bobby Fischer to the latest champion, IBM's "Deep Blue".

1 … e5

The classical "Double King-Pawn Opening". Both sides establish a foothold in the center, while allowing their king bishops to develop.

2 Nf3

The most preferred move because it develops with a threat – to Black's king-pawn. Other good moves here are 2 Bc4 (the Bishop's Opening) and 2 Nc3 (the Vienna Game) as each represents sound development. Less advisable for young players are 2 d4 (the Center Game), because after 2 … exd4 White has to retake with the queen, and 2 f4 (the King's Gambit), as Black plays 2 … exf4 and although White will have a free hand regarding advancing the d-pawn, it will be a struggle to recover the gambitted pawn.

2 … Nf6

This move, known as Petroff's Defense, after the 19^th century Russian master, is fundamentally a counterattack on White's king-pawn. This is a sound defense, but Black must follow-up exactly. Players should always be wary of copying the opponent through a counterattack, because of the great choice which the original attacker will enjoy.

3 Nxe5 Nxe4?

The correct move for Black here was 3 … d6, because then after 4 Nf3 Nxe4 5 Qe2, he can both defend the knight and break the e-file pin with 5 … Qe7. After the bold but incorrect 3 … Nxe4, White has several appealing possibilities.

One is 4 Qf3, which constitutes a double attack against the knight on e4, and the square f7, as the main threat is 5 Qxf7 mate. To successfully counter a double attack of this type, a move which defends both threats is necessary. Here Black has several such moves. He can play 4 … Nf6, both retreating the e4 knight and denying access to f7. He can play 4 … Ng5, again retreating the knight, but this time defending f7 directly, as well as attacking the White queen on f3! Also a double defense, although less appealing for strategical reasons, is 4 … Nd6 – the knight is safe while f7 is protected, although Black's development will be stunted because the knight interferes with the development of Black's bishops. Finally, Black has less good tactical double defenses such as the ineffective 4 … f5?! (as White can just reply 5 Qxf5) and 4 … Qe7, which protects f7 and plans to meet 5 Qxe4 with 5 ... d6, which recovers some of the material.

4 Qe2!

But a better choice for White on his fourth move is 4 Qe2! This move introduces the special trap which awaits Black if he copies White's moves. The copying cannot continue long after 4 … Qe7 5 Qxe4 as then 5 … Qxe5?? just loses the queen to 6 Qxe5+. This is a dramatic illustration that the copying routine must at some point come to a sudden end. After 4 Qe2, Black also cannot retreat his knight; for example, 4 … Nf6? and then White can deliver a mean discovered check on the king-file just by moving his knight. Investigation shows that the best place for White's knight to visit with this free pass is c6, as from that square White's knight attacks the Black queen while the Black king is in check from White's queen. Thus, 5 Nc6+ would win Black's queen. Paradoxically, after 4 Qe2, Black's best is actually the copying move 4 … Qe7, but here the point would be to meet 5 Qxe4 with 5 … d6, which wins back the knight (due to the pin on the e5 knight to the queen on e4) but will end up a pawn down.

4 … d5

This might be the second best move on the board (best was 4 … Qe7 5 Qxe4 d6). Black holds the knight in position on e4.

5 d3!

It is important to attack immobile pieces. Here Black's knight cannot move due to the menace from the threatened discovered check by White's knight. Notice that 5 d3 is much stronger than 5 f3, which would have allowed a counterattack with 5 … Qh4+.

5 … Qe7!

The most useful way to hit the knight on e5. If 5 … Bd6 instead, then after 6 Nf3, White would win the Black knight which would then be fully pinned. This is why 5 … Qe7 was necessary – as now 6 Nf3 would be met very simply by 6 … Nf6.

6 dxe4 Qxe5 7 exd5

White has done a lot of hard work, but it has been worth it – he has emerged from the complications one pawn to the good.