Double check
A double check is a single move that puts the enemy king under attack from two pieces simultaneously, typically one that moved and one it unmasked, so only a king move can answer it.
The rulebook says a check must be removed, and there are three ways to do it: capture the checker, block its line, or move the king. A double check shuts the first two doors. You cannot capture two pieces with one move, and a single block cannot cover two different lines. Only the king can respond.
That is why a double check is so frightening. If the king has no safe square, the game ends on the spot, and these mates are often the neat final blow of a combination. If the king can run, the move still dictates everything, because the opponent has no choice about the reply and you know where the king lands.
Most double checks come from a discovered check where the piece that moves also gives check from its new square. So the way to search for one is to start from your discovered check ideas and test each landing square of the mover for a check of its own. Knights make ideal movers, because a knight check lands from a distance.
To protect yourself, give the king breathing room with luft, avoid standing in a line behind your own piece when an enemy rook or bishop is aimed there, and watch your king's flight squares before pieces crowd around it.
In the diagram White plays Nc7#. The knight checks from c7 and uncovers the rook on e1, so the king on e8 is attacked twice. The rooks on d8 and f8 and the pawns on d7 and f7 box the king in.
It sits close to Blackburne's mate. Blackburne's mate is a checkmate made with two bishops and a knight, where a bishop checks on h7 guarded by a knight on g5 while the other bishop on the long diagonal covers g7 and h8.
A neighboring idea is Greek gift sacrifice. The Greek gift sacrifice is a bishop sacrifice on h7, or h2 for Black, played as Bxh7+ Kxh7, followed by a knight check on g5 and the queen coming to the h-file to attack the exposed king.
Also worth reading is Royal fork. A royal fork is a fork that attacks the enemy king and queen at the same time, most often with a knight, so the king must move and the queen is lost.
To see how often "Double check" or a pattern like it decides your own games, book a first lesson: George Li reviews your games and writes a study plan around what he finds.
Related terms
A discovered check is a check from a piece that did not move: a bishop, rook or queen is unmasked when the piece in front of it leaves the line to the king.
In a discovered attack one of your pieces steps off a line, and that move unmasks a bishop, rook or queen standing behind it, so a single turn produces two threats.
A double attack is one move that creates two threats at once, whether they come from a single piece or from two, and whether they aim at pieces or at checkmate.
A royal fork is a fork that attacks the enemy king and queen at the same time, most often with a knight, so the king must move and the queen is lost.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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A game review, a read on your level and a written study plan from George Li.
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