Bizarre pin rule: pinned pieces do not attack 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?
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 first 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.