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   The Forsyth-Edwards Notation (FEN for
  short), is the standard notation or method for describing Chess positions or
  a particular board position of a Chess game. It was created in 1883 by newspaper
  journalist David Forsyth from Scotland and slightly extended later by an
  American computer scientist Steven J. Edwards for use in computer Chess
  software. The main purpose of FEN notation files is to provide all the
  information required to restart a game from a desired Chess position. Typical FEN string showing the Chessboard
  initial position: 
 
 
 
 FEN strings contain separators between
  fields using blank spaces: 
 Field #1: Chess
  pieces setup: This field number 1 is comprised of 8
  strings or sections of letters and numbers separated by slashes ( / ): 
 Each string or section represents a rank
  or a row of 8 squares of the Chess board running from left to right and starting
  from the uppermost section of the board (black pieces). Every rank is filled
  up with letters taken from the standard English names: “P”= Pawn, “N”=
  Knight, “B”= Bishop, “R”= Rook, “Q”= Queen, “K”= King, and numbers from 1 to
  8. White pieces are identified by uppercase letters (PNBRQK) and black pieces
  by lowercase letters (pnbrqk). Adjacent or contiguous empty squares are
  identified with a single digit number (1 to 8), e.g.: a whole empty row of 8
  squares should be represented by an “/8/”, not as “/1/1/1/1/1/1/1/1/” and
  this also applies to any other lower amount of contiguous empty squares in a
  row. The following image is an alternate way
  of visualizing the board with the above FEN file: 
 
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References
http://www.chess-poster.com/english/learn_chess/notation/notation.htm