Pin. A bit like a pen, but there’s an i. Weird. Also, there are there things called PIN-codes. Maybe this is enough for an intro.
Pins are everywhere in chess. They are one of the most common tactical motifs, and many other motifs and combinations rely on pins at some point. But what is a pin?
When a more valuable piece is behind another piece that is attacked by a not so valuable piece (a bishop, a rook or a queen), then the piece in front is in a pin, or pinned. It could also be that the back-piece isn’t defended, and if the front piece moves, the back piece will be captured. The idea of a pin is anyways, that the front piece cannot move without exposing the back piece.
There are absolute pins, where the back piece is he king. In this case the front piece cannot move at all, because if it moved, the king would be in check. That’s illegal, and illegal moves send you to jail. Or lose you the game anyways.
Here’s a few absolute pins. None of black’s pieces or pawns can move because of the absolute pins. He can only move his king to either h7 or h8.
There are relative pins. They are pins, where he back-piece isn’t a king, but is typically more valuable than the attacking piece. Here is an example of a relative pin, where the black knight is pinned by the bishop. If the knight moves, black would lose his queen for a bishop, which is a bad trade. The queen could leave the pin by moving to d6 for example still protecting the knight.
There are also partial pins, where the pinned piece can still move in the same row, line or the diagonal as the pin, but cannot leave it. Here the back bishop is partially pinned by the white bishop, but the pin is pretty weak, because black can just trade the bishops by Bxg3. Anyways, the situational pins idea is seen here, because the bishop can’t move to a1 for example without exposing the queen, but I can move along the white bishop’s diagonal without exposing the queen behind it.
And finally, there are situational pins, where the piece isn’t really pinned, but moving it would cause terrible things to happen. Here it’s white’s move, but he can’t save his knight by moving it to safety, since the rook would checkmate the king in the next move, for example if Ne5, Rd1#. This is called a back-rank checkmate, where the white pawns block the white king’s movement and a rook gets to the back-rank to mate him. That’s why it’s often a good idea to push one of the pawns forward especially in an endgame, so that he king can move to safety.
There are some typical pin-ideas that occur in many games. In this one the white bishop can win a rook by moving Bh6. The pawn can’t take the bishop, because the queen pins it to the king. It’s an absolute pin by the way. To avoid getting checkmated by Qxg7#, the only sensible move is g6, which exposes the rook for capture. The pawn was actually also pinned by the bishop as a relative pin. Funny, isn’t it?
As a conclusion, pins occur a lot in games and you have got to know the motif to be aware to them. You don’t have to learn all the pin types, but it can help with talking about the game