Passed pawn
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In chess, a passed pawn is a pawn as to which there are no opposing pawns to stop it from advancing to the eighth rank, i.e. no opposing pawns in front of it on the same file nor the files to either side. A passed pawn is sometimes called a passer. In the diagram at right, the white pawns on b5, c4, and e5 (see algebraic notation) are passed pawns. Black's pawn on d4 is a passed pawn. If Black plays fxg4, then Black will also have a passed pawn on g4, and White will have a passed pawn on f4. Passed pawns are an advantage because an opponent's piece must intervene (often by sacrificing itself) to stop them from promoting.
A striking (albeit very unusual) example of the power of passed pawns is seen in the position at left, the conclusion of a problem by Leopold Mitrofanov. Black, with a queen, bishop, and knight, is helpless against White's two passed pawns, which threaten both 10.b7# and 10.c8(Q)+ Bb8 11.b7#. If 9...Qd5, 10.c8(Q)+ Bb8 11.b7+ Qxb7 12.Qxb7#; if 9...Qg6 10.c8(Q)+ Bb8 11.Qb7#; if 9...Qa5+, 10.Kxa5 Kb7 11.bxa7 and Black cannot stop both pawns.
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[edit] Protected passed pawn
A passed pawn that is protected by its own pawns is called a protected passed pawn. In the first diagram in this article, the pawns on the b and e files are protected passed pawns. Two or more passed pawns on adjacent files are called connected passed pawns (see connected pawns), and they are very strong. In both the first and second diagrams, White's b and c pawns are connected passed pawns. A pair of connected passed pawns is sometimes called a steamroller. It is often strategically advantageous for the side with connected passed pawns to place them on the same rank and then advance them in tandem, because this makes them more difficult to blockade.
[edit] Outside passed pawn
An outside passed pawn is a passed pawn that is on or near the left or right edge of the board, and is separated by a number of files from the rest of the pawns, e.g. the pawn on the a file in the diagram at left. Such a pawn often constitutes a strong advantage for its owner. In the diagram position, even though material is equal, White's outside passed pawn will divert Black's king away from the center, leaving White's king free to "mop up" Black's pawns and win the game. White wins with 1.a5 g5 2.a6 h5 3.a7 Kb7 4.Kd5 e4 3.fxe4 fxe4 5.Kxe4 and the king will win Black's remaining pawns.
[edit] Passed pawns in the endgame
Passed pawns are particularly important, often of decisive significance, in the endgame. The position at left provides a dramatic example of this. White has no passed pawns and seems to be in desperate straits, since Black's king will soon attack White's pawns with ...Kg4. In fact, White by means of a sacrificial combination creates a passed pawn and wins: 1.g6! fxg6 (or 1...hxg6 2.f6! gxf6 3.h6!) 2.h6! gxh6 3.f6! and White's newly created passed pawn will queen. If it is Black's move, he must avoid this combination by playing 1...g6! (not 1...f6 2.h6!, nor 1...h6 2.f6!).
Since passed pawns have no opposing pawns to stop them, the threat of queening often forces the opponent to use a piece to block or capture the pawn, wasting valuable time and possibly losing material. Indeed, a far-advanced passed pawn or pawns is often equal to or even superior to a piece. Four examples of this are seen in the diagram at right. In the upper-left quadrant of the board, White's connected passed pawns on the sixth rank are superior to Black's rook. Even if on move, Black cannot stop one of White's pawns from queening. Similarly, in the upper-right quadrant, Black's bishop cannot hold back both of White's pawns. White queens a pawn after 1.f7 (1.h7 also works) Bxf7 2.h7 followed by 3.h8(Q). In the lower-left quadrant, White's queen cannot stop Black's pawn from queening without stalemating Black. The lower-right quadrant highlights how awkward a knight is in dealing with a passed pawn, especially a rook pawn. White's knight is actually worse than useless in trying to stop Black's pawn. It cannot do so itself, and if White's king (which could catch the pawn if the knight were not there) approaches with 1.Kf2 (hoping for 1...hxg2? 2.Kxg2), Black plays 1...h2! and 2...h1(Q).