Pin vs skewer: which piece is in front
The two tactics use identical geometry and are constantly confused, including by people who have been playing for years. The distinction is one word long.
The difference in one sentence
In a pin the less valuable piece is in front, so it cannot move without exposing the bigger piece behind it. In a skewer the more valuable piece is in front, is attacked directly, and moving it gives up whatever is behind. Pin restrains; skewer collects.
Both are attacks along a rank, file or diagonal by a bishop, rook or queen, against two enemy pieces that happen to be lined up with nothing between them. Neither can be executed by a knight or a pawn, because both require a line that passes through the first piece to reach the second.
The practical difference is timing. A pin does not win anything by itself -- it freezes a piece so that a second attacker, or a tactic somewhere else, can do the winning. A skewer usually wins material immediately, because the front piece has no way to stay where it is and the piece behind it is simply collected.
One useful consequence: a pinned piece is a defender that is not really defending, so after you pin something, look again at what it was supposed to protect. A skewer needs no such follow-up thinking. It is already the tactic.
One of each
Pins
Position 1
White to move. Find the best move.
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27...Bd7 28.Re8#
The line ends in checkmate. The piece in front is worth less than the piece behind it, so it is frozen and whatever it was defending is effectively undefended.
The position comes from an endgame of a real game, rated 850 for difficulty. The answer reaches checkmate.
Skewers
Position 2
Black to move. Find the best move.
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40.f5 40...Rxg2+ 41.Kf4 41...Rxg7
The line wins a rook. The piece in front is worth more, so it has to move, and the piece behind it is taken.
The position comes from an endgame of a real game, rated 610 for difficulty. The answer reaches a winning advantage.
Questions people ask
What is the difference between a pin and a skewer?
Which piece is in front. A pin attacks a less valuable piece that is shielding a more valuable one, so the front piece cannot move. A skewer attacks the more valuable piece directly, forcing it to move and exposing the piece behind it.
Is a skewer just a reverse pin?
Effectively, yes, and it is sometimes called exactly that. The geometry is identical -- one line, two enemy pieces, no piece between them -- and only the order of value differs.
Can a knight pin or skewer?
Neither. Both tactics require an attack that travels along a line and continues through the first piece to the second. A knight's attack does not travel along a line at all.
Which is stronger, a pin or a skewer?
A skewer, usually. It wins material immediately, whereas a pin creates a restraint that still has to be converted into something. The exception is the absolute pin against a king, which can paralyse a defender for the rest of the game.