A FEN string consists of six fields separated by spaces. The first field describes piece placement rank by rank from the eighth rank to the first, using uppercase letters for White pieces (K, Q, R, B, N, P) and lowercase for Black (k, q, r, b, n, p), with numbers indicating consecutive empty squares. A slash separates ranks.
The remaining fields specify: the active color (w or b), castling availability (KQkq or combinations thereof, or - if none), the en passant target square (or - if none), the halfmove clock (moves since the last capture or pawn advance, for the fifty-move rule), and the fullmove number (starting at 1 and incrementing after Black's move).
FEN is invaluable for sharing specific positions in analysis, puzzles, and discussions. Rather than describing a position verbally or showing a diagram, a FEN string conveys the complete position in a compact, unambiguous format that any chess software can parse and display. The starting position FEN is: rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1.