Bizarre pin rule: pinned pieces do not attack

Question

I found this guy who likes playing chess, but not enough to bother reading the rules; he only plays using a physical board with his sister (she doesn't know the rules either).

The made up rule was this:

A pinned piece is not attacking the squares he can't reach the next move without exposing the king.

Which means, a lot of things can happen:

• A king can move to an 'attacked' square if the piece is pinned.
• By removing the pin, the piece now attacks the squares he wasn't (this can result in discovered checks if the king was in one of these squares).
• You can remove a check by pinning the attacking piece.

If you think about it, it requires much more thinking process to play like this. I was at a disadvantage because it is really hard to see these new threats and tactics if you never played like this before.

My questions:

• Does anyone know if this is some kind of known chess variant?
• Should I tell them the correct rules even when I know they are not that serious about playing?
• Are these rules more complex to understand than the official pin rules?

Answer 1

Well, it's a variant (since it's played by your friend and his sister, and probably at least some others), but not a well-known one. I used to think this idea would be only logical until I realized that a King moving into a square attacked by a pinned piece could still be captured by the pinned piece, and even though the opposing King would then be exposed to attack, the other side would already have lost. Note that though Black leaves himself in check with his move, he still wins because he theoretically captures the King first:

[FEN "8/8/5k2/4r3/8/2B5/8/5K2 w - - 0 1"]

1.Ke1 Rxe1 2.Bxf6

Compare with a position in which it's a mate in one for whichever side moves first. Say White checkmates, then Black has lost even though he can checkmate in one move. This is due to the same reason that Black won in the previous diagram. Theoretically, White wins because he can capture the opposing King first:

[FEN "6k1/5ppp/6r1/8/8/1R6/PPP5/1K6 w - - 0 1"]

1.Rb8# Rg1# 2. Rxg8 Rxb1

To your second question, I see no reason why you should not let them know the real rules. You are the best judge of whether he'd be open to learning them. Don't be overbearing and certainly be willing to play his variant if he obviously doesn't care for the standard rules. Games are for fun.

And to the extent that it requires consideration of more possibilities, it is more complex tactically.

Answer 2

Answers

• It sure is a chess variant, and now that you brought it up, it just became of the known kind. I take good note of it, even though I never heard of it before. It does sound illogical, as Daniel's comment highlight, so I don't think regular players would come up with it.

• Tell them or not : does it really matter ? Don't be authoritative about the standard ruleset, that would be pointless. If you can see a way to lightly mention the official rules, do it, maybe they care, but if they don't, that's it.

• These rules are illogical once one understands it's all about capturing the king, as Daniel very well points out. Yes, they are more complex to understand.

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Strange things happening

Here's a position where I think this ruleset allows strange things to happen :

[FEN "8/8/2Rr1k2/4n3/3B4/3K4/8/8 b - - 0 0"]

1... Nxc6 {Is moving an actually pinned piece out of it's pinning direction legal ?}

Here, the white rook pins the black one, so the bishop is not pinned, but the knight is. Altogether, white pins black. Black to play, capturing the rook would lead to a legal position, because it indirectly pins the bishop, a posteriori freeing the knight, for there is no check from the just-pinned bishop. I argue this being a legal move is dubious, for the knight is pinned, for the time being.

It could be in this ruleset, but I found that to make these rules highly paradoxical. Somehow, you shouldn't be able to get a pinned piece out of its pinning direction — even if later on, it comes back on the board to legitimate it had the right to go away. I feel there's a “meantime” in which the position isn't legal, hence the move shouldn't be.

Let's develop on that “delay” thing :

[FEN "8/3rPK2/2k5/8/8/8/8/8 w - - 0 1"]

1. e8=P {what now ?} (1. e8=B {legal}) (1. e8=R {promoting to a non-bishop-behaving piece is illegal})

If the newborn consensus is right — a move ending up with a legal position is legal — then this promotion can only be made to a bishop-behaving piece. Again, I'm not liking the time during which the pawn lets the king vulnerable, but I'm more and more convincing myself that that's just my pinning intuition getting in the way.