Passed pawns

It is a wonderful sight for any chess player when he/she sees his/her opponent squirm and uncomfortably try to stop a passed pawn. Worth just a single point, but they should rather be thought of as potential queens(in most cases), especially when the endgame nears. Great endgame players such as Vassily Smyslov, Jose Raul Capablanca, and Magnus Carlsen noticeably excel in this area of the game because of the value they place on creating a passer. In the endgame, a mating attack is very rare and usually does not succeed, unless the opponent has a poorly placed king with a weak pawn structure, etc. Hence, the difference between a win and a draw can only be made apparent by marching your pawns up the board.  Carlsen, in particular, is known for his pressing chess in the middlegame, which he often implements by pushing pawns up the board and gaining some space. For example, he likes pushing the a pawn to a4 and a5(and the reverse for black) to prevent and queenside expansion by his opponent. While this is useful in the middlegame, its real strength is demonstrated in the endgame when the king or a rook snaps up the blocked pawns. The key here is that since the pawn is already so far advanced, it will be hard to stop along with the king.
Carlsen-Anand, World Chess championship, 2013. Here white's pawn on a5 is a cramping influence and leaves black with a paralyzed queenside. Despite the hanging knight, white can create a passed pawn for himself. However, black has counterplay with his knight on the kingside so the match ended with queens being traded off(draw).

Assistance to passed pawns:
Passed pawns do not get to the 8th on their own. They need to be supported well. Queens are usually the best, but are usually traded off by the time passed pawns are able to be created. Hence rooks should be placed behind the pawn to support it well. Placing it ahead of the pawn will impede the pawn's progress. The king should be right alongside the pawn so that it can kick any opposing obstacle away.
Passed pawn strengths:
Two or even three passed pawns are almost unstoppable as not only are they free to romp down the board, but they support each other as they progress.Two united pawns can defeat a rook as well. An important strategy during the middlegame is sacrificing an exchange to get a pawn and clear the path for a pawn so it may advance. This is done because rooks tend to be clumsy in middlegames with a lot of maneuvering so a sacrifice like that could very well take an opponent by surprise. This has been adopted by players such as Veselin Topalov.
What piece to use?
 Knights are generally the worst pieces to assist passed pawns and even to stop enemy ones. Due to their property of controlling one square from exactly one other square, they can easily be ejected from their defensive post. Bishops are extremely effective when it comes to passed pawn support. They should generally cover the queening square so that no piece can block the pawn's advance.
Other properties:
 A pawn will usually not be allowed to reach the 8th rank unless the opponent commits a serious blunder or the player produces a bit of genius. The enemy will use most of the resources available to him to prevent the pawn from queening, and so passed pawns are generally used as distractions. In simple king and pawn endgames, the mere location of a pawn can win the game, even if material is equal. An outside passed pawn is one that is located a little away from the action or towards the edge of the board which induces the opponent's pieces to get distracted and try to stop it. While they are doing this, you can snap up their undefended material. This is perhaps the most important use of passers.
Conclusion:
Endgame technique is a fine art and requires precision. It relies on logic more than any other factor. Players should recognize the importance of creating a passed pawn and should think of it as a potential new piece. This can make a draw into a win.
Note: It is not necessary that a pawn must always become a queen. In specific positions, making the pawn another piece is actually required. This is called underpromoting. It may be done to check the opponent's king when your own king is in danger. It is also usually done to avoid stalemate and other tricks.

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