Sacrifice (chess)
In chess, a sacrifice is a move giving up a piece with the objective of gaining tactical or positional compensation in other forms. A sacrifice could also be a deliberate exchange of a chess piece of higher value for an opponent's piece of lower value. Any chess piece except the king may be sacrificed. Because players usually try to hold on to their own pieces, offering a sacrifice can come as an unpleasant surprise to one's opponent, putting them off balance and causing them to waste precious time trying to calculate whether the sacrifice is sound  or not, and whether to accept it. Sacrificing one's queen (the most valuable piece), or a string of pieces, adds to the surprise, and such games can be awarded . Types of sacrifice. Real versus sham. Rudolf Spielmann proposed a division between sham and real sacrifices: In compensation for a real sacrifice, the player receives dynamic, positional, or other non-material advantages which they must capitalize on, or risk losing the game due to the material deficit. Because of the risk involved, real sacrifices are also called "speculative sacrifices". Sham sacrifices. The tactical sham sacrifices can be categorized further by  the mechanism by which the sacrifice is made. Some sacrifices may fall into more than one category. Other types of sacrifices. Forced versus non-forced. Another way to classify sacrifices is to distinguish between "forcing" and "non-forcing" sacrifices. The former type leave the opponent with no option but acceptance, typically because not doing so would leave them behind in material with no compensation. Non-forcing sacrifices, on the other hand, give the opponent a choice. A common error is to not recognize when a particular sacrifice can be safely declined with no ill-effects. Examples. Deflection sacrifice. In the  diagram, GM Aronian has mistakenly played 24. exd4, opening up the e-file for Black's rook. After Svidler played 24... Re1+, Aronian resigned, because Black's move forces the reply 25.Rxe1 (or 25.Qf1 Qxf1), after which White's queen is undefended and therefore lost. This particular type of sacrifice has also been called the "Hook and Ladder trick", for the white queen is precariously at the top of the "ladder", while the rook is at the bottom, supporting it. Suicide sacrifice. Black played 1... Qxg3 and White drew with 2. Qg8+! Kxg8 (on any other move Black  will get mated) 3. Rxg7+!. White intends to keep checking on the seventh , and if Black ever captures the rook it is stalemate. This save from Evans has been dubbed "The Swindle of the Century". White's rook is known as a desperado. Non-forcing sacrifice. This time Reshevsky is at the receiving end of a sacrifice. White has just played h2–h4. If Black takes the knight he has to give up his own knight on f6 to avoid mate on h7. Instead, he simply ignored the bait and continued developing. Positional sacrifice. In this game Black played  14... d4! 15. Nxd4 Nd5. In exchange for the sacrificed pawn, Black has obtained a semi-open file, a diagonal, an outpost on d5 and saddled White with a backward pawn on d3. The game was eventually drawn. Sacrifice to checkmate. The following example features a forced bishop sacrifice by White. White can force mate in two moves in the diagram at left as follows: hxg6 2. Qxg6 Queen sacrifice leads to smothered mate. In this position, Black moves 22... Qg1+ forcing the white rook to take black's queen by 23. Rxg1; the king cannot take the  queen because it would have been in check from the knight on h3. Having forced the rook out of a position where it was defending the f-file and into a position where it blocked the king from making any move, the black knight delivers a smothered mate by 23... Nf2#.