README.md

@@ -5,6 +5,30 @@ Build a clean, fast chess engine in C that starts from a FEN string and produces
 ## Approach
 - **Correctness-first engine:** `FEN → board → legal movegen → PERFT`
 
+## FEN (Forsyth–Edwards Notation)
+
+FEN is a single line string that describes a chess position. Format:
+
+    <pieces> <side> <castling> <en-passant> <halfmove> <fullmove>
+
+- **pieces**: 8 ranks (8→1) separated by `/`; letters for pieces (`PNBRQK`/`pnbrqk`), digits for empty squares.  
+- **side**: `w` (White) or `b` (Black) to move.  
+- **castling**: any of `KQkq` or `-` if none.  
+- **en-passant**: target square after a two-step pawn move, or `-`.  
+- **halfmove**: moves since last pawn move/capture (50-move rule).  
+- **fullmove**: move number (starts at 1, increments after Black’s move).
+
+**Example (starting position)**
+
+    rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
+
+- pieces: starting layout  
+- side: `w` (White to move)  
+- castling: `KQkq` (both sides can castle both ways)  
+- en-passant: `-`  
+- halfmove: `0`  
+- fullmove: `1`
+
 ## Future Work
 - Incorporate algorithms and evaluation methods for chess positions to enable an **AI player**.