King Safety & Check

A king is in check when it is attacked by one or more opposing chessmen.

That turns ordinary attack relationships into a compulsory legal condition:

a player whose king is in check must make a legal move that removes the check.

The player cannot postpone the problem, ignore it, or make an unrelated move.

What check is

Check is a game state.

It means:

• the king is currently attacked;
• it is that side's responsibility, when having the move, to produce a legal resulting position in which its king is no longer in check.

Check is not:

• a capture of the king;
• a material score;
• an optional warning;
• the end of the game by itself.

The game ends only if the check is checkmate, which requires that no legal response exists.

Checkmate is developed in 02.3.

Check means the king is attacked. Checkmate means the king is attacked and no legal move can solve it.

The king is never captured

The rules do not continue until one player physically captures the opposing king.

Instead:

• leaving one's own king under attack is not allowed;
• exposing one's own king to attack is not allowed;
• capturing the opposing king is not allowed.

A correctly played game ends at checkmate before any king capture can occur.

Check exists whether or not anyone says "check"

The legal state comes from the position.

It is not created by a verbal announcement.

A player may notice the attack instantly or may fail to notice it, but the rule condition is the same.

For editorial simplicity:

• diagrams should label check when teaching the concept;
• gameplay should never imply that the word itself is required to make the attack legally real.

How check can arise

Check can appear in several ways because many kinds of legal move can change attack relationships.

Direct checking move

A chessman moves to a square from which it directly attacks the opposing king.

Examples:

• rook moves onto an open file with the king;
• bishop moves onto a checking diagonal;
• knight moves to a checking destination;
• queen moves onto a checking line;
• pawn moves so its diagonal attack reaches the king.

Check geometryDifferent chessmen can create the same legal state: check

RRook

BBishop

NKnight

QQueen

PPawn

Capture / attack

Checking capture

A chessman captures an enemy piece and, from the destination square, attacks the king.

The captured chessman may also have been the blocker that previously prevented the line.

Discovered line

A chessman moves away from a line and reveals an attack from:

• rook;
• bishop;
• queen.

If the revealed line attacks the king, the resulting position is check.

Opened lineMoving a blocker can reveal a check behind it

BeforeBishop blocks

→

AfterOpened-line check

Blocked / unavailableCapture / attack

Promotion can give check

A pawn reaching its promotion rank is replaced as part of the same move by:

• queen;
• rook;
• bishop;
• knight.

The new chessman's effect is immediate.

Therefore a promotion can produce check immediately.

The full promotion rule belongs to 02.6.

En passant can change lines

An en-passant capture removes:

• the capturing pawn from its old square;
• the captured pawn from a different square;

and places the capturing pawn on the passed-over square.

Because two occupied squares change, an en-passant move can open or close a line involving a king.

The full rule and its king-safety edge cases belong to 02.5.

Castling can give check

Castling moves:

• the king;
• one rook.

The rook's new square can create a rook attack on the opposing king.

Castling remains one move.

The full castling rule belongs to 02.4.

Check is determined by the resulting attack map, not by a special category of "checking moves."

The legal obligation when in check

A player in check must remove the check with a legal move.

There are three broad response families:

1. move the king;
2. capture the checking chessman;
3. block the checking line, when blocking is geometrically possible.

Not every check allows all three.

The response is legal only if the resulting king is safe.

A response must solve the entire check

Suppose the king is checked by a rook.

You cannot:

• move another chessman somewhere unrelated;
• attack the opponent's queen;
• win material elsewhere;
• make your own threat;

unless that move also removes the check legally.

When in check, "stronger threat" is not a fourth response category.

The move must first produce a position where your king is not in check.

If a checking response also creates a counter-threat, that is secondary.

Response 1 — move the king

The king may move to an adjacent destination only if the move is legal.

The destination must satisfy:

• normal king movement;
• no friendly chessman occupies it;
• if an enemy chessman occupies it, that chessman can be captured there only if the final square is safe;
• the destination is not attacked by the opponent.

Escaping checkEvery king destination must be classified in the resulting position

friendlyrooksafesafebishop

White to move

CorrectIncorrectBlocked / unavailable

The king cannot flee into another attack

Escaping the original checker is not enough.

Example:

• rook checks along the e-file;
• king moves off the e-file;
• destination square is attacked by an enemy knight.

The move is illegal.

Escape from the checker is not the test. Safety in the resulting position is the test.

The opposing king attacks squares too

A king may not escape to a square adjacent to the opposing king.

Even if no rook, bishop, queen, knight, or pawn attacks the destination, the opposing king's adjacency attack makes the move illegal.

This is the mutual-king rule from 02.1 applied during check.

Capturing with the king

If the checking chessman stands adjacent to the king, the king may sometimes capture it.

But only if the destination square is not attacked after the capture.

This means a defended checking piece may be impossible for the king to capture.

Capturing the checkerThe king may capture only if the destination is safe

AUndefended checker

White to move

BChecker defended

White to move

CorrectIncorrect

Response 2 — capture the checking chessman

The checker may sometimes be removed by:

• the king;
• another friendly chessman.

The capture is legal only if the resulting position leaves the king safe.

Capture by another chessman

Example:

• Black bishop checks White king along a diagonal;
• White knight can capture the bishop.

If after NxB:

• the bishop is removed;
• no other Black chessman attacks the White king;

the check has been legally resolved.

Check responseA checking slider can sometimes be captured by another chessman

checker

White to move

Capture / attack

Pinned defender cannot always capture

A friendly chessman may geometrically attack the checker but be unable to capture it legally because moving would expose its own king to another attack.

This is why:

• attack map;
• legal move list

must remain separate.

Pinned defenderA geometric capture can be illegal if the defender is pinned

pinnedchecker

White to move

Focus squareCapture / attack

Capturing one checker is not enough if another remains

If the king is attacked by two chessmen, removing only one attack does not solve the position.

This leads to double check later in this section.

Response 3 — block the checking line

Some checks travel along a line between a sliding attacker and the king.

Those checks can sometimes be blocked.

Possible line-checking chessmen:

• rook;
• bishop;
• queen.

To block:

• a friendly chessman moves to a square between checker and king.

InterpositionA sliding-piece check can be blocked when an intermediate square exists

BeforeOpen e-file

White to move

→

AfterBe3 blocks

block

Capture / attackCorrect

Blocking does not capture the checker

A block leaves:

• checking chessman on the board;
• king on its square;
• new blocker in between.

The line no longer reaches the king.

That is enough to remove the check if no other attack remains.

Blocker must itself produce a legal position

If the blocking chessman is pinned elsewhere or the block opens another attack on the king, the move can still be illegal.

Again: resulting king safety is the final test.

Checks that cannot be blocked

Blocking requires a square between the checker and king.

Therefore several check types cannot be blocked.

Knight check

The knight does not attack along an intervening line.

There is no square where a blocker can be inserted.

Possible responses:

• move king;
• capture knight;

if legal.

Pawn check

A pawn attacks an adjacent diagonal square.

There is no intervening square.

Possible responses:

• move king;
• capture pawn;

or another legal resource that removes the attack.

There is no line to block.

Adjacent king check

In a legal game, kings cannot move into mutual adjacency.

So a legal position cannot arise with one king simply standing adjacent and checking the other after a legal move.

This is better treated as illegal king movement rather than a normal blockable check example.

Contact rook/bishop/queen check

If a sliding checker is directly adjacent to the king along its attack line, no square exists between them.

Therefore the check cannot be blocked.

Interposition geometryA line check is blockable only when a square exists in between

ABlockable

BNo interposition square

CorrectCapture / attack

Summary

You can block a line only when there is a square between checker and king.

Double check

A king is in double check when two opposing chessmen attack it simultaneously.

This commonly arises when:

• a moving chessman gives check;
• that same move also uncovers a second line check.

At the rules level, the key consequence is:

the king must move.

A non-king response cannot solve two simultaneous checks by merely:

• capturing one checker;
• blocking one line.

The legal move must remove the king from both attacks.

Double checkOne bishop move can create two simultaneous attacks on the king

BeforeRook line blocked

→

AfterBc6+ · double check

Blocked / unavailableCapture / attack

Double check also has rich tactical uses, but the rules-level consequence is the essential point here: the checked king must move.

Checking the legality of every response

When evaluating a proposed response to check, use a complete result test.

Step 1 — identify every checker

Is the king attacked by:

• one chessman;
• more than one?

Step 2 — choose a response family

Can you:

• move the king?
• capture the checker?
• block the line?

Step 3 — make the move conceptually

Update:

• moved chessman;
• captured chessman;
• opened/closed lines;
• destination occupancy.

Step 4 — recalculate attacks on the king

After the move:

• is the king still attacked by the original checker?
• is another attacker revealed?
• is the king destination attacked?
• did a pinned chessman move illegally?
• did an en-passant/capture line change matter?

Step 5 — accept only a safe resulting position

If the moving side's king is still in check, the response is illegal.

Check-response legality

• [ ] Identify all current checking chessmen.
• [ ] Proposed move follows movement/occupancy rules.
• [ ] Any capture/block is correctly executed.
• [ ] Original check is removed.
• [ ] No second check remains.
• [ ] No new line attacks the king.
• [ ] King destination, if moved, is not attacked.
• [ ] Resulting position leaves own king safe.

Check, threat, and checkmate are different

Threats, checks, and checkmates are related but legally different concepts.

Threat

A harmful consequence may happen next if ignored.

Check

The king is attacked now.

Checkmate

The king is attacked now and no legal move removes the attack.

Check practice

Identify the checker

Practice with five basic check types:

1. rook check;
2. bishop check;
3. knight check;
4. queen check;
5. pawn check.

For each position, identify the checking piece and describe the attack geometry that reaches the king.

Choose the response family

For each check, test the three broad response families:

• move the king;
• capture the checker;
• block the checking line.

Compare these cases:

1. a long rook check where all three may be possible;
2. a knight check, which cannot be blocked;
3. an adjacent queen check, which has no interposition square;
4. a bishop check where the checker cannot be captured;
5. a double check, where only a king move can work.

Legal king capture?

Compare two adjacent checking pieces:

A. the checker is undefended; B. the checker is defended by another enemy piece.

The king may capture only when its destination square is safe after the capture. The deciding factor is whether that square is attacked, not simple adjacency.

Pinned defender

Consider a friendly piece that attacks the checker geometrically but is pinned to its own king.

Question: Can it capture the checker?

Answer: No, if moving that piece would leave the king in check.

Check rules at a glance

The rules of check can now be compressed into one model:

1. A king is in check when attacked by one or more opponent chessmen.
2. A player may never leave or place their own king in check.
3. When in check, the next move must remove all attacks on the king.
4. The broad response families are:

• move king;
• capture checker;
• block a checking line.

1. Not every check permits every response.
2. King destinations must be unattacked.
3. A checking chessman may be impossible for the king to capture if its square is defended.
4. Pinned chessmen may still attack squares even when their own legal move is constrained.
5. Double check requires a king move.
6. Check is not checkmate.

If no legal response exists, ordinary check becomes checkmate—a separate game-ending state with its own exact test.