Double Check
A double check is a discovered check where the piece that moves also gives check on its own — two attacking pieces, two separate lines, both landing on the king at the same instant. It only takes one extra ingredient to turn a discovered check into a double check, but that ingredient removes an entire category of defense that every other kind of check still allows.
The one thing a double check takes away
Against a normal check — even a discovered one — the defending side usually has up to three options: move the king, block the line, or capture the checking piece. A double check eliminates two of those three at once. Blocking is impossible because there are two separate lines and no single square interrupts both of them. Capturing is impossible for the same reason — even if one of the two attackers could technically be captured, the other one is still giving check afterward. Moving the king is the only response left, every single time, which makes a double check the most restrictive type of check that exists.
Starting position. A knight move creates two checks at once (illustrative position, not from a recorded game).
The knight both checks the king directly and clears the way for the bishop — two lines of attack from one move, and the king has no way to answer either one.
Why this makes double check so dangerous
Because the king absolutely must move, a double check works even in positions where the opponent has plenty of material sitting right next to the king that would ordinarily be able to help — none of it matters, because none of it can be used. This is what makes double check the engine behind some of the most abrupt swings in chess: a king that looked completely safe, surrounded by defenders, can be forced into a losing square with zero say in the matter, because every one of those defenders is suddenly irrelevant to the one move being played.
Recognizing the setup in advance
A double check requires two things to line up: a piece of yours (very often a knight, since its move pattern lets it attack a king directly from an unusual angle) sitting in front of a second long-range piece of yours that's aimed at the same king, blocked only by the piece about to move. The knight doesn't need to be planning a double check when it develops — the pattern usually appears suddenly, once the board opens up enough for the piece behind it to have a clear line. It's worth specifically re-scanning for this whenever a knight of yours is both near the enemy king and sitting in front of one of your own bishops, rooks, or queen.
What usually follows a double check
Because the king is forced onto a square it didn't choose, a double check is frequently the first move of a forced mating sequence rather than the whole tactic by itself — the king ends up somewhere with even fewer escape squares than before, and the attacker follows up immediately while the defender is still purely reacting. Even when it doesn't lead to mate, forcing the king to an objectively worse square for free is usually worth more than whatever material the checking pieces could have won by attacking something else instead.
Every double check is technically also a discovered check, but not every discovered check is a double check — the difference is entirely about whether the piece that moves attacks the king too, and it's worth checking for specifically, since it's the difference between the king having one legal reply and having none of the usual three.
Frequently Asked Questions
Can a double check ever be blocked?
No. Blocking works by interposing a piece on the line between a single attacker and the king, and a double check has two separate lines by definition — a piece placed on one line does nothing about the other. This is true regardless of what pieces are involved or how the position is set up.
Is a double check always checkmate?
No — the king only has to have a legal square to move to for the game to continue. Double check is extremely forcing, not automatically fatal; whether it's mate depends entirely on whether every square around the king is covered or occupied, the same as with any other check.
Why does a double check often come from a knight rather than a bishop or rook?
A bishop or rook that moves can only ever check the king itself along a straight line, which usually means it wasn't blocking anything in the first place. A knight can attack a king from a square no straight-line piece could reach, which is exactly the kind of odd angle that lets it deliver check on its own while also stepping off a second piece's line — the two conditions overlap far more naturally with knight moves than with any other piece.