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Patterns

The 10 tactical patterns detected by ChessGrammar, with definitions and examples.

ChessGrammar detects 10 tactical patterns. Each is defined by structural conditions on the board and confirmed through forcing tree analysis (L2).

Fork

A single piece attacks two or more enemy pieces simultaneously. The opponent can only save one, resulting in material gain.

  • Key square: Where the forking piece lands
  • Targets: The attacked pieces (2+)
  • Common pieces: Knights (most frequent), queens, pawns

Example — Knight forks king and queen:

FEN: 6k1/5p1p/4p3/4q3/3n4/2Q3P1/PP1N1P1P/6K1 b - - 3 37

curl -X POST https://chessgrammar.com/api/v1/extract \
  -H "Content-Type: application/json" \
  -d '{"fen": "6k1/5p1p/4p3/4q3/3n4/2Q3P1/PP1N1P1P/6K1 b - - 3 37"}'

Pin

A piece is immobilized because moving it would expose a more valuable piece behind it to attack. The pinned piece may still be able to move along the pin line.

  • Key square: The pinning piece's position
  • Targets: The pinned piece and the piece behind it (usually king or queen)
  • Pin line: Rank, file, or diagonal connecting the three pieces

Types: Absolute pin (against king, piece cannot legally move) vs. relative pin (against queen/rook, piece can move but at material cost).

Skewer

The reverse of a pin: a high-value piece is attacked along a line, and when it moves, a piece behind it is captured.

  • Key square: The attacking piece's position
  • Targets: The front piece (forced to move) and the piece behind it
  • Common pieces: Bishops and rooks along open lines

Discovered Attack

A piece moves, revealing an attack from another piece behind it. The moving piece may also create its own threat (discovered check is the most powerful variant).

  • Key square: The piece that moves (the blocker)
  • Targets: The piece attacked by the revealed attacker
  • Variants: Discovered check (revealed piece gives check), discovered double attack

Double Check

Two pieces give check simultaneously — the moving piece and the revealed piece. The king must move; it cannot block or capture both attackers.

  • Key square: Both checking pieces
  • Targets: The enemy king
  • Note: The most forcing type of check. Often leads to rapid checkmate.

Back Rank Mate

Checkmate delivered on the back rank (1st or 8th rank), where the king is trapped by its own pawns and cannot escape vertically.

  • Trigger: A rook or queen delivers check on the back rank
  • Condition: The king has no flight squares (typically blocked by f/g/h pawns)
  • Note: One of the most common checkmate patterns in practical play

Smothered Mate

Checkmate by a knight where the king is completely surrounded by its own pieces. The knight's unique jumping ability makes it the only piece that can deliver this type of mate.

  • Trigger: Knight delivers check on a square adjacent to the king
  • Condition: All king escape squares are blocked by friendly pieces
  • Classic example: Philidor's smothered mate (Qg8+ sacrifice, then Nf7#)

Deflection

A piece is forced away from a critical defensive duty, allowing a tactical blow. The deflecting move typically captures or threatens the defender.

  • Key square: Where the deflecting move lands
  • Mechanism: The defender is lured/forced away from protecting a key square or piece

Interference

A piece is placed on a critical square to block a defensive line between two enemy pieces. This cuts off communication or protection.

  • Key square: The interference square
  • Mechanism: Blocking a rank, file, or diagonal that an enemy piece uses for defense
  • Difference from deflection: The defender stays in place but its line is blocked

Trapped Piece

A piece has no safe squares to move to and will be captured. The piece's mobility is reduced to zero useful squares.

  • Condition: Every legal move of the piece results in capture for less or equal value
  • Common victims: Bishops trapped on the rim, knights on the edge, rooks blocked by pawns