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up vote 5 down vote favorite

I have a question and I'm sorry if it sounds very uninformed, I'm not a professional chess player so my interest is mere curiosity. I was wondering if there are examples of illegal positions that require more than a superficial look at the chessboard to be recognized as illegal.

All examples of illegal positions that come to my mind are pretty trivial.

  • A missing king
  • Two kings of a color
  • Too many pieces considering possible promotions (e.g. 4 queens + 6 pawns)
  • Pawns in the first rank
  • Pawns in the eight rank
  • All pawns in the second rank and other pieces in higher ranks (except knights)

It's very easy to see that these examples are all illegal. Are there any illegal positions that require more insights to spot?

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2  
This would be a great plot devise in a Bond movie/novel. – br1ckb0t 11 hours ago
1  
Are positions that are unreachable, but otherwise follow all rules, considered illegal? – immibis 9 hours ago
    
@immibis It seems that "All pawns in the second rank and other pieces in higher ranks (except knights)" is illegal only because it is unreachable, no? – Dougal 8 hours ago
    
You forgot a few trivially illegal cases, e.g. 1) Both kings are in check (including kings next to each other). 2) King in check by more than 2 pieces. 3) King in check by combinations of 2 pieces that could not result from double check. 4) Pawns in low ranks and far from home file (e.g. white pawns at e2, f2, g2, h2, e3, f3, g3, h3 is impossible). 4) Pawns far from home file without enough opponent pieces captured. 5) Too many bishops of same color (player and square) accounting for promotions 6) Extensions of your last rule - some pawns have moved, some pieces in unreachable spots. – Meni Rosenfeld 1 hour ago
up vote 13 down vote

Yes - the less a position looks like a real chess game, the harder it is to spot if it is illegal or not. Sometimes, retrograde analysis is needed to prove a position can be reached in a legal way. For an example, see the starting position of the Horse Concoction, which can be proven to be legal:

[FEN "8/7P/1P5B/2B1Q1n1/3nn2P/1PRnk1nR/3nnnK1/2B1nQBn w - - 0 1"]

However, as White's pawns have to have captured at least five black pieces, the same position with an additional black piece is illegal:

[FEN "8/1b5P/1P5B/2B1Q1n1/3nn2P/1PRnk1nR/3nnnK1/2B1nQBn w - - 0 1"]
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up vote 4 down vote

If we define "illegal position" as a position that cannot happen in a game using legal moves, I think most difficult to spot would be positions that look normal, but are impossible to achieve.

[FEN "2k1rr2/pbpq2b1/1p1p1np1/nB1Pp2p/2P1Pp2/2N2NP1/PPQB1PP1/2KR3R w KQkq - 0 1"]

For example this position looks legal, but indeed there is no way how this position could happen in real game - all 16 black pieces are on the board and yet white have doubled pawn. There could be lots of such positions in which none of the rules you mentioned are broken and yet, the positions are impossible to achieve, for example: 

    [FEN "4rrk1/ppp1bppp/2npbn2/1B6/3NP1q1/1P1Q1N2/P1P2PPP/1K1RR1B1 w Qq - 0 0"]

There is no way white bishop could get to g1

    [FEN "2krr3/pppbppbp/2n1pnp1/1B2q3/4P3/2N1QB2/PPP1NPPP/R4RK1 w KQkq - 0 0"]

There is no way how white could get two white-square bishops. Yes they could promote a pawn theoretically, but the only square where this would be possible is d8 and this square is black. So had they done so the bishop would be black-squared (not mentioning the fact that with two pawns on c7 and e7 there is no way how bishop could get away from there.

    [FEN "6k1/p1p1np1p/1p4p1/3Kqr2/8/BQP3P1/P4P1P/1R6 w - - 0 0"]

What was the last move? etc.

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up vote 3 down vote

One of the most irregular rules says you cannot castle to escape check. You must move your king, block the check, or capture the checking piece. So, if a king is in check and castles it would not be visually so noticeable.

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Thanks @MarkH. It would be great if you could add a real board as an example. – seawalker 17 hours ago
1  
It's not at all obvious how you could construct an example of this. The castling may not have been the last move. – David Schwartz 12 hours ago

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