How to make illegal positions

I came up with that it is legal.

don't look at the above puzzle.

how did that come about? illegal?

*my bad*

There is supposed to be a white knight at a1 (or anywhere else for that matter). There was in the position I had set up OTB, and somehow it never made it into the position I put on this site.

Considering Ihow much fail I've already posted, I should probably double check my puzzles before I post them.

Anyways, HERE is the correct position:

why? i don't know why this is illegal...

So here's the solution I thought of (SPOILER):

Assume the position is legal, meaning it is a possible position in a game beginning with the standard chess setup and following the rules of chess. White's pawns left on the board have of course captured five black pieces with axb, cxb, exd, fxe, and exd. This leaves only one more black piece to capture and only the g and h pawns to promote. Both of these must have promoted to dark square bishops, because white's initial dark square bishop (on c1) can never have moved, since the b and d pawns are still on their home rank. The only way to do this by capturing just one piece is gxh, capturing the h pawn and moving the g pawn to a file with a black promotion squre. However, for this to be possible, black's pawns on g6 and h6 must have come from f7 and g7, respectively, capturing two white pieces. Neither of these could have been the dark square bishop (again, because it never moved), so they must be the white knight and light squre bishop (two rooks, a knight, and the queen are still on the board). But since white must have captured two black pieces on each of the b and d files, and since the only black pieces except one rook are pawns, at least one black pawn must have moved from its original file onto the b or d file, capturing another white piece. This means white started with an extra piece, contradicting our assumption that the game began in a standard chess setup. Thus our assumption is false, and the position is illegal.

wow?

Solution to my very ignored puzzle!

so white must have blocked the rook earlier.

and now you see there are many many moves possible for black! so it is legal.

It is an interesting puzzle.  :)  While it is true that black had a few options before white put him in double check, it is still possible that the position is illegal.  I'll look at it some more tomorrow.  I'm not 100% sure yet if it is legal or illegal.  :)  I'm leaning towards illegal because there are so few captures and three promotions are needed.  This means that black has never captured a white pawn and c8, e8, and g8 are the only possible squares to promote to a light bishop.  Black's d-pawn is a passed pawn which required at least one capture to make it a passed pawn.

As far as I know, the only rigorous way to prove a position is legal is to provide a possible legal game (or legal continuation from a position known to be legal) that results in that position. In this sense, it is often easier to prove a position illegal than legal.

@ SerbianChessStar

a little subtler of an idea...

(if it's already here then sorry I don't read 160 posts...)

Illegal.

Can't figure out a way to make it legal, given that knights cannot lose a tempo and the rooks are restricted to only one movement square...

You can also make illegal moves by doing a PGN format like this:

1. O-O O-O-O 2. Kh1 Kb8 3. O-O-O O-O 4. O-O-O O-O 5. Kg5 Kh6 6. b8=Q dxb8 7. O-O-O

and it will turn out like this:

The first one of Hotflow (post #52) was actually legal too...

I found this one somewhere:

Hint One:  Think about white's last move.

Hint Two:  The bishop can't be moved to a4.

Hint Three:  The king was on b3.

Hint Four:  If the king was on b3, how would the double check happen?

Here is the solution (don't read until you found the answer) :

Yes, it is a legal postion.  The king was on b3 in a double check.  The only way this can happen is by an En Passant capture.  So, it started out with the king on b3, a black pawn on b4, and a white pawn on c2.  White was in check by the bishop.  White played c4 and after bxc3 En Passant, Kxc3 and we arrived at the diagramed position.

Your puzzle looks similiar to the puzzle in this forum.  :)

yes, that's actually the actual problem "Where is the king", but I just asked if it was legal