A skewer is an attack along a line where the more valuable piece stands in front. When it moves out of the attack, a less valuable piece behind it is captured. It is the mirror image of a pin, and like pins it can only be executed by bishops, rooks, and queens.
The skewer is the pin's aggressive twin. In a pin, the valuable piece hides behind a lesser one and you attack the shield. In a skewer, the valuable piece stands in front — it must step aside, and whatever sheltered behind it is taken.
An endgame, heavy pieces traded down. Black's king stands on e5, and Black's rook lingers on e8 — same file, careless geometry. White slides a rook to e1: check. The king must move off the e-file, and the rook on e8 falls. That is the whole tactic: check, step, capture.
Skewers against the king are the most forcing kind, because the front piece has no choice about moving. Queen-behind-king skewers decide endgames constantly — especially in pawn races, where a freshly promoted queen gets skewered against her own king by the older queen along a diagonal.
Skewers punish two pieces standing on the same line with the wrong one in front. That alignment rarely happens by force; it happens by drift — a king wandering onto its rook's file, a queen parked on the same diagonal as the king. Strong players develop an almost physical discomfort seeing their own king and queen aligned, even with pieces between them, because lines have a way of opening.
The defensive habit is therefore geometric: after every move, notice which of your pieces share a rank, file, or diagonal, and whether an enemy bishop, rook, or queen could exploit that line. This is the same line-awareness scan that guards against pins and discovered attacks — one habit, three tactics covered.
If capturing the front piece is the point, it's a pin (the back piece keeps it in place). If the front piece will flee and you win the one behind, it's a skewer. Same geometry, opposite value order.
Missed skewers — both yours and your opponent's — show up clearly when you analyze your own games. Chessdock extracts those moments automatically and lets you replay them as puzzles until the alignment jumps out at you in real time.