A pin tactic and a skewer tactic are visually very similar. In both cases you attack two targets on the same diagonal, rank or file. However, there is one very important difference between a pin and a skewer.
To understand the subtle but important difference between a pin and a skewer it may be useful to first understand exactly what a pin is–and then compare that to what a skewer is.
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Example of a Pin Tactic
In the example below black moved their bishop to f6 and pinned white’s knight on c3:
The important observation here is that the value of the knight is lower than the value of the rook behind it. In other words, in the case of a pin–the attacked piece can’t move because it would expose a higher-valued target behind it.
What is a skewer tactic in chess?
In the case of a skewer, the attacked piece has a higher value than the target behind it. This is why it’s also sometimes referred to as a “reverse pin”.
Example of the difference between a skewer and a pin
The 2 diagrams below will illustrate the difference between a skewer and a pin:
In the case of a pin, the pinned piece has a lower value than the target behind it. But in the case of a skewer the piece at the front in the line of attack has a higher or similar value to the piece behind it.
In many situations, although not always, a skewer is more powerful than a pin–because the threat is very direct, whereas in the case of an ordinary pin the threat is often not as serious and your opponent has more time to deal with a pin.
The grey area between a pin and a skewer
What happens when the attacked piece has the same value as the piece behind it? Would it then be a skewer, or a pin? The answer isn’t clear because this scenario is partly a skewer and partly a pin. Shall we call it a “pewer”? 😀
Summary: The difference between a pin and a skewer
Even though a pin and a skewer visually appear quite similar, their impact on the position is very different.
So to summarize the difference between a pin and a skewer: A pinned piece shouldn’t move because it would expose a higher-valued target behind it. A skewered piece should move, even though it will expose another target behind it, because the skewered piece has a higher value than the target behind it.