Diagonal (Chess) - Definition & Use
Diagonal
Definition
In chess, a diagonal is any straight line of squares that extends in a 45-degree angle across the board, connecting squares of the same color. Formally, two squares lie on the same diagonal if the difference in their file letters equals the difference in their rank numbers (e.g., a1–h8 or h1–a8). There are 26 distinct diagonals on an 8×8 board: 13 on the light squares and 13 on the dark squares.
How the Term Is Used
The word “diagonal” appears constantly in chess writing and conversation. Typical contexts include:
- Piece mobility: Bishops and queens control or “open” a diagonal when no piece blocks their path.
- Positional planning: “Placing the bishop on the long diagonal” often means fianchettoing it on g2 or b2 (or …g7/…b7 for Black).
- Tactics: “Exploiting the h1–a8 diagonal” may refer to a mating attack on the long dark-squared diagonal after moves like Bc3 or Qh8#.
- Blockade and obstruction: Pieces can be pinned or skewered along a diagonal.
Strategic Significance
Diagonals are the lifelines for bishops—the only piece that must stay on them. Controlling a key diagonal can decide entire games:
- The Long Diagonals: a1–h8 (light-square) and h1–a8 (dark-square) influence all three sectors of the board. A well-placed fianchetto bishop often paralyzes the opponent.
- The “Spanish” diagonal: b1–h7 (or its mirror) is critical in the Ruy Lopez; White’s Bc4 or Bb5 pressures f7/f6 via this line.
- Central diagonals: c2–g6 and c7–g3 are common avenues for the Greek Gift sacrifice (Bxh7+ or Bxh2+).
Illustrative Examples
1. Fianchetto Power (King’s Indian Attack)
After 1. e4 d6 2. d3 Nf6 3. Nf3 g6 4. g3 Bg7 5. Bg2 O-O, White’s bishop on g2 stares down the a8 square. One thematic continuation is 6. O-O c5 7. Nc3 Nc6 8. h3 Rb8, and the entire middlegame revolves around who controls that a1–h8 diagonal.
2. World-Champion Spotlight
In Fischer – Petrosian, Candidates 1962, Fischer’s dark-squared bishop landed on d5, slicing along the b1–h7 diagonal and freezing Petrosian’s king side. Although Petrosian eventually escaped, the pressure illustrated how one invading bishop can dominate a game.
3. “Diagonal of Death” – Kasparov vs. Topalov, Wijk aan Zee 1999
When Kasparov’s knight landed on e6, his queen swept the a2–g8 diagonal. The ensuing cross-fire produced one of history’s most celebrated combinations, finishing with the famous quiet move 24. Qh8+!!. The queen and bishop battery across the diagonal left Black helpless.
Common Tactical Themes Along Diagonals
- Pin: Bc4 pins the f7-pawn to the king on e8.
- Skewer: A queen on a1 skewers king and rook on the a1–h8 diagonal after promotion tactics.
- Discovery: Move a knight on e4 to unleash a bishop on b1 toward h7.
- Deflection: Sacrificing on f7 can remove the king from the diagonal’s protection line.
Historical Notes & Trivia
- The term fianchetto comes from Italian for “little flank” and almost always refers to deploying a bishop onto a long diagonal (g2, b2, etc.).
- Before 1475, bishops (then called “alfil”) leapt exactly two squares diagonally, severely limiting diagonal play. The modern sliding bishop transformed chess strategy.
- Some annotators speak of “laser bishops,” “sniper bishops,” or the “dragon bishop” to dramatize the power of a piece on an open diagonal.
- In scholastic circles, the mnemonic “bishops like to go for long walks on the diagonal” helps beginners remember their movement.
Key Takeaways
Mastery of diagonals equals mastery of bishop and queen power. Whether you are locking one down with pawn chains or prying it open with pawn breaks like f4 or c4, ask yourself each move: “Which diagonals am I strengthening, and which am I weakening?” Answering that question correctly often separates strong positional play from casual piece shuffling.