En passant

En passant is the one capture in chess where the piece taken is not on the square the capturer lands on. When a pawn advances two squares from its starting square and stops beside an enemy pawn standing on its own fifth rank, that enemy pawn may take it as if it had advanced only one: it moves diagonally onto the square the pawn skipped, and the pawn that advanced is removed. A white pawn on e5 answers ...d7-d5 with exd6; a black pawn on d4 answers c2-c4 with dxc3. The right exists on the very next move, and never again.

What makes it a tactic, and not just a rule, is where the captured pawn disappears from. After exd6 the capturing pawn stands on d6, but the pawn it took vanishes from d5, a square the capturer never touched, so any line running through d5 opens at once: a queen or a bishop behind that pawn is suddenly giving check or hitting a piece. En passant is also the fastest way to change a pawn structure: it undoes a blockade, breaks a chain, and can turn a pawn that was running past yours into a passed pawn of your own. That last one is why the rule exists: the two-square first move was added to chess later, and en passant stops a pawn from using its extra speed to slip past a square an enemy pawn controls.

To spot it, every time your opponent pushes a pawn two squares, look at the squares on either side of where it landed. If one of your pawns stands there, en passant is on the board for exactly one move. Then ask what the capture does besides winning a pawn: which line opens behind the pawn that vanishes, what the capturing pawn attacks from its new square, and whether it is now a passed pawn. Players forget en passant far more often than they misjudge it, and a double step past your pawn is often a sign that your opponent forgot it too.

At higher level it is a defensive resource and a legality check. When a pawn's double step gives check, taking it en passant can be the answer, and if it is the only legal move it is compulsory like any other. In pawn endings it decides races: count the capture as a move that removes the runner and advances your own pawn a rank. And it can be illegal: if taking would expose your own king, en passant is not allowed. The classic case is a king and an enemy rook on the same rank as the two pawns: the capture removes both pawns from that rank at once and opens the rook's line to the king.