Every chess player who wishes to improve himself needs to practice his ability to count variations and
combinations - he needs to practice his chess tactics.
Here comes an example : Suppose WHITE starts the game with 1.e4.
If now BLACK replies 1.d5, WHITE can capture the pawn at d5 with 2.exd5.
This is a variation, involving the move sequence 1.e4 d5 2.exd5.
WHITE seems to have gained some material, but BLACK is able to capture back at d5 by 2.Qxd5, restoring the material balance.
This again is a variation (1.e4 d5 2.exd5 Qxd5). A variation may have any number of moves.
He does so in order to capture later. If now WHITE plays 3.Nc3 to protect the pawn, BLACK may still equalize the material after 3.Nxd5, ready to meet 4.Nxd5 with 4.Qxd5.
But what happens if WHITE supports his pawn with 3.c4 instead of 3.Nc3? Now BLACK can not capture the d5-pawn for WHITE will capture the knight and eventually WHITE will have captured a knight and a pawn (worth 3+1=4), while BLACK only two pawns (worth 1+1=2).
This material difference is enough to state that BLACK will lose the game, no matter how well he will play from here on, provided that WHITE will not blunder anything either.
This means that, after the first move of the combination, all the variations that may arise depending on the opponent's move, will lead to an advantage.
Note that when counting variations and combinations the player should account for the best opponent's reply, not only some of them that favor him.
Had he seen WHITE's threat on f7, he could have comfortably defended against it, say by 3.Nf6. Indeed, WHITE's move 3.Qf3 was not good (early Queen move); it was just a trivial trap.
Had BLACK played correctly, he would have been better off a few moves later.
The exclamation mark denotes a very good move and the cross symbol denotes a check).
The move 4.h6? is a mistake, for it permits WHITE to apply the winning combination which starts with 5.Nxe5!.
Now WHITE threatens to mate by 6.Bxf7+ and 7.Nd5 and at the same time he threatens the Bishop on g4. BLACK could have defended now with 5.Be6, covering both threats and escaping the worse. But 5.Bxd1?? is a blunder that allows WHITE to mate.
He could have done better had he tried 4.Qe7, for if now WHITE captures the Ne4 with 5.Qxe4, BLACK can respond 5.d6 and he will capture back. After 6.d4 dxe5 7.Qxe5 WHITE has an advantage due to the extra pawn.
Accuracy in counting the variations is what makes a chess player a brilliant tactician. It is not always easy to count all the variations; besides there are so many possibilities.
However, only a handful of moves are actually of importance in most positions; the rest can be ruled. Experience helps players quickly select candidate moves.
Playing against computers (yet not ever getting disappointed) is highly recommended in order to improve one's tactics capabilities.
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