legal AND illegal positions

Yup. You got it :D

Legal.

The last response to this thread before yours was over a year ago...

That depends on your credentials. To reach the position you must have been drinking big time (together with your opponent). At the wrong age this is totally illegal.

Edit: Sorry, it is my seven year itch.

Curious.

This is very difficult to do; that's why there are retrograde analysis problems that ask about the past of a position (such as "is this position legal").

Legal or illegal?

Illegal, the pawns could not pass each other without capturing

Good point.

Legal or illegal?

Oops, I meant with nine queens. Edited it.

Legal or illegal?

Even when you add 9 black queens, it is still legal!

Can you come up with a series of moves ending in that position?

I could but I won't. I will however describe how it works:

Take pairs of pawns by both sides for instance on the a- and b-file. By making 2 cross captures of pieces you can promote 4 pawns. When you do that 4 times across the board you end up with 16 promoted queens and 8 captured pieces. This is no problem since 12 units are available to be captured when you leave the kings and queens on the board!

Legal or illegal?

Then show a series of moves reaching that position.

White is missing both rooks. Black is missing the light-squared bishop. White made one obvious capture (exf), Black made one obvious capture (bxc). Since White must capture Black's light-squared bishop, exf is really e2xf3.

What happens before and after that?

First, note that before e2xf3, White's light-squared bishop is stuck on f1. Now, clearly Black must make b7xc6 before White makes e2xf3 (to get the bishop out fom c8 to f3). Before b7xc6 itself, White must free a rook out to get captured on c6. However, f1, g2, h2 are all occupied by White's pieces, so White's kingside rook cannot get out; thus it's White's queenside rook that gets captured on f3.

Since White is only missing one capture, there is no cross-capture; all pawns (except those mentioned) remain on their files. How does White's queenside rook get out?

Recall that f1 is occupied by a stuck bishop. a2, d2, e2 are clearly pawns that never move. So White's rook must get out via b2-b3 or c2-c3. But since there is no cross-capture, simply moving the pawn b2-b3 isn't enough. So White's pawn moves c2-c4 before White's rook can get out.

Moreover, if the pawn on b2 is not moved yet, White's dark-squared bishop is stuck on c1, so White's rook is also caged. Thus b2-b3 must be done before White's rook can go out.

But now observe what's happened. The pawns a2, b3, c4 are there before White's rook goes out. After White's rook goes out and gets captured, the pawns c6, d7 are there. These form a wall on the left side that cannot be passed by the light-squared bishops. Thus Black cannot even get his light-squared bishop captured; White also cannot get his light-squared bishop to the left side.

So the position is illegal.

You've read "The Chess Mysteries of Sherlock Holmes"? Or some other retrograde analysis book? I knew the position was illegal; I wanted to see who else would realize.

I thought you composed that position.

This book is indeed a great one. There are many veteran retro solvers in these forums though that have seen and solved far more advanced retros than anything found in the book. There are some very nice compositions laying around, too.

Legal or illegal?