Illegal Position Contest!

Wouldn’t this be a draw?

I think….how did the black king get into that single safe space?

That looks so easily legal. Do you think it's illegal?

You don't have to guess about it. Just play obvious moves to make a proof game, then you'll know it's legal.

Anyway, the point of the position I gave you to prove illegal was to show that when you're deciding about legality, a good way to start is to check whether both sides have enough available captures to get their pawns in place.

No. The men are the 16 units each side starts with, eight pawns and eight pieces.

What I said means, for example, if White has seven men on the board, whether pawns or pieces, Black has made exactly nine captures. If the black pawns still on the board would need ten captures to arrive at their places, the position is illegal.

In the position I gave you to check, Black had 12 men on the board, so White had made four captures. But the white pawns on the board would need five captures to reach their places.

No. Read what I just said again. Black has eight pawns and four pieces on the board, a total of 12 men, so White has made four captures. Now count how many captures by white pawns have to occur to reach the white pawn position. It adds up to five captures, one more than White has available.

Right!

Wha-what!? I’m so confuzzled

Well, very obvious, this is a illegal position...

But you got it in the end. Do you see why it's captures by pawns that need to be counted? A piece can move to a new file without capturing anything, but a pawn has to make a capture to reach a new file. So looking at a player's pawn position can give you a lower limit on how many captures have been made.

Counting captures by pawns will sometimes let you prove a position is illegal, but will never let you prove it's legal. But before I start trying to make a proof game, I check the other things that typically make a position illegal, such as,

1. An obvious illegality, like a pawn on the first rank, a side with more than eight pawns, a king in triple check, both kings in check, more promoted pieces than missing pawns, etc.

2. A ninja bishop on the board. (That's what I call a bishop that could only reach its position by jumping over another piece.)

 3. A promoted bishop that couldn't have promoted on the right color square.

4.  A last or second last move that can't be legally retracted (usually applies when the side to move can be inferred, such as when a king is in check).

5. One side has an abducted pawn. (That's a pawn that couldn't have promoted or been captured and is no longer on the board.)

6. One side runs out of tempo moves before the other side can achieve its final position. (May require a tough retroanalysis to release a tricky position.)

If none of those problems are found, the position might be legal and it's time to try making a proof game.

A Queen or a Bishop en passanting a pawn

You've already seen an example of #5 at https://www.chess.com/forum/view/fun-with-chess/illegal-position-contest?page=294#comment-64689503

For #3, see if you can explain what makes this position illegal:

What in the...

But there’s another wrinkle to it, right? The white pawn could have promoted on d8 to become a DSB, but to do so, it would have had to capture a d-pawn itself. In order to do that, black would had doubled pawns on the d-file at some point. But in order to have doubled pawns, black would have needed to capture something of white’s. Because all of white’s pieces are in play, it was impossible for black to have ever had doubled d-pawns. Because it was impossible for black to have ever had doubled d-pawns, white couldn’t have promoted on the d8-square.

EDIT1:

Scratch that. White has a knight out of play.

Isn’t that a legal position? White plays Nd6 at some point, black plays …exd6 to get doubled pawns, and then white marches the e-pawn up and captures d6. After that, white promotes to a DSB and then retreats it. All that can happen, yes?

EDIT2: Also scratch that -- black has doubled f-pawns, so it was impossible for white's e-pawn to have captured anything of black's to change files to promote on a dark square.

Anyway, I was thinking up positions last night. I haven't read all 300 pages of this topic, so I have no idea if has been done before and is trivial at this point, but here's one.

How many black men are still on the board? How many have been captured?

EDIT2

That one might be a little too easy. Black has no legal retraction to uncheck the white king.

Illegality reason #4 at https://www.chess.com/forum/view/fun-with-chess/illegal-position-contest?page=297#comment-64963705