A double attack is any move that creates two threats at once — against pieces, mate squares, or promotion — so the defender can only deal with one of them. The fork is its best-known form, but double attacks are broader: the two threats can come from different pieces entirely.
If you reduced all of chess tactics to a single principle, this would be it: create two problems, receive one answer. The double attack is that principle in its purest form, and almost every tactical device — forks, discovered attacks, many deflections — is a specific way of manufacturing one.
A queen swings to d5 in a position where Black has neglected development: it eyes the f7 pawn next to the king — threatening a quick mate — and simultaneously attacks the undefended rook sitting on a8, all along two different diagonals. Black must address the mate threat. The rook is collected next move.
Note what made it work: one threat was lethal (mate), the other target was loose. Two mild threats don't make a double attack — the defender just picks the cheaper one to allow. The currency is forcing-ness: at least one threat must be too expensive to ignore.
A fork is a double attack delivered by a single piece attacking two targets. The general double attack is wider: a move can unleash two threats from two different pieces — you move a knight, and now the knight attacks a rook while the bishop it unblocked attacks the queen. That particular flavor has its own name, the discovered attack, and it is the most violent of the family because the two threats genuinely arrive in the same instant.
Search order matters. Start from the targets, not the moves: list your opponent's loose pieces, exposed king squares, and overworked defenders — then ask which of your moves touches two items on that list. This is dramatically faster than checking every legal move for consequences, and it is the practical core of tactical threat-spotting.
Defensively, the same list read in reverse tells you what your opponent is hoping for. Keeping your pieces defended and your king sheltered doesn't just avoid individual tactics — it starves double attacks of targets altogether.
Chessdock mines your own games for the double attacks you missed and the ones that hit you, then drills you on those exact positions — because the fastest tactical improvement comes from your own recurring blind spots.