In chess, a tactic refers to a short sequence of moves which limits the opponent's options and which results in tangible gain. Tactics are usually contrasted to strategy, in which advantages take longer to be realized, and the opponent is less constrained in responding.
A single chess move considered in isolation is below the level of tactics. To take an enemy piece or deliver check may be useful, but unless it is part of a plan, a move is usually not classified as a tactic.
The fundamental building blocks of tactics are two-move sequences in which the first move poses a double threat. The opponent is unable to respond to both threats in one move, so the first player realizes an advantage on the second move. This class of tactic includes forks, skewers, discovered attacks, undermining, overloading, and interference. Pins also fall into this category to some extent, although it is common for a defending player to relieve neither of the two threats posed by a pin, in which case the attacking player commonly maintains the pin for a longer period of time. A pin is therefore sometimes more strategic than tactical.
Often tactics of several types are conjoined in a combination. A combination, while still constraining the opponent's responses, takes several moves to obtain advantage, and thus is considered deeper and more spectacular than the basic tactics listed above.
The concept of zwischenzug is often listed as a tactic, but might properly be called a counter-tactic instead. During the execution of a tactic one focuses on a only few pieces as relevant, but a zwischenzug complicates the situation by making a more urgent threat with another piece. The effect of a zwischenzug is to change the status quo before a tactic can come to fruition. The near ubiquity of the zwischenzug makes long combinations all the more rare and impressive.
Chess instructors usually steer beginners away from any detailed study of openings, focusing instead on tactics and endgames, which serve as the basis for later strategic understanding. One should not suppose, however, that one's understanding of tactics is ever completed. It is not the case that all master players know everything there is to know about tactics, and differ only in depth of strategic understanding. On the contrary, as Garry Kasparov famously asserted, a grandmaster (GM) can often tactically overwhelm a mere international master (IM).
Chess computers are considered superhuman at tactics and rather weak at strategy. The fact that computers can play on a par with the best humans suggests that chess is primarily a tactical game. On the other hand, it must be noted that computers don't think about tactics in human terms (fork, skewer, etc.); the nuances of human understanding of chess, both tactical and strategic, have not been imitated by computers, only matched in effective playing strength.
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