With the rook on a1, 18. Rxa7 isn't best anyway, instead 18. Qd6 wins the bishop on c6 (as other kibitzers have pointed out). I didn't consider it because c2 is hanging, but after 18...Qxc2+ 19. Re2+ wins the queen.

17.Rxa7!? 0-0! 18 Rxa8 Rxa8 19 Rxe7 may well win, but better is the direct 17.Rxe7+ Kf8 18.Rxa7. Time to check:
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That extra Rook sure does help!

Here's a Crafty End Game Trainer link to the extra-Rook variation 17.Rxa7!? 0-0! 18 Rxa8 Rxa8 19 Rxe7 with White about to play 19 Rxe7 to which Crafty EGT will reply Qc3 to leave

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And, no, I didn't write that. It's a quote from the first series of the Twilight Zone - seems somehow appopropriate for today's POTD.

Let's see if we can sort all of this out.

First, a message for those of you watching from the future. And no, I don't mean a distant future where the polar ice caps have collapsed and the earth is ruled by (a) apes (b) killer robots or (c) 1970's style punks wearing uncomfortable trousers and driving modified cars with spikes stuck all over them whilst trying to kill a young Mel Gibson and waiting for Tina to sing.

Although, of course if anyone is watching from a future like this - hello! Good luck.

No, this is for folks watching from the near future and wondering why were are talking about a rook on a1. You see, that's because CG.com inadvertently published the puzzle with the aforementioned major piece sitting on the aforementioned bottom left hand corner, like this:

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In fact, Philidor played this game at rook odds so there never was a rook on a1. But we've inadvertently created an alternative universe where that rook exists, so we might as well solve the puzzle. If indeed it is a puzzle. If a tree falls over in a forest and no-one hears it, does it make a sound? I don't know, but I do know that if a bear was standing next to the tree whilst it was falling over, that bear would probably have an involuntary bowel movement.

From our hypothetical twilight zone puzzle position, you might be tempted by 1. Rxa7, thinking that black cannot reply 1...Rxa7 because you could play 2. Qd8#. But this would throw away white's advantage: 1. Rxa7 0-0 2. Rxa8 Bc5+.

No, the way to win our twilight zone puzzle is 1. Rxe7+ Kf8 2. Rxa7

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White has a huge advantage and should win very soon.

Return to normality, and the puzzle without the Ra1. The game score shows what happens if black rolls over (metaphorically) to have his metaphorical tummy metaphorically tickled. But Fritzie wants black to build a fortress with: 17. Rxe7+ Kf8 18. Qd6 Kg8 19. Qxc6 Rf8

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And we get one of those strange positions where black has very few threats of his own but is hunkered down behind barricades. It's kind of like a weird alternative future where some kind of a-pocky-lips has happened, nearly all the oil has run out and so, rather counter-intuitively, everyone has taken to driving cars with immense engines through the Australian desert. And in the middle of all this madness a group of good honest decent people have somehow stumbled across an oil well and have fortified themselves in said oil well whilst the baddies in the previously-referenced uncomfortable trousers try to get in. As you do.

Could we save you 15 percent on your auto insurance? Does a bear ____ in the woods?

In this case, does it make it difference if the extra Rook isn't on the a1 square? I suggest it does.

For example, in today's Tuesday puzzle solution, it makes a difference after 17. Rxe7! Kf8.

If the Rook is missing, as in the position below,

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the only clear winning continuation is 18. Qd6! when play might continue 18...Kf8 19. Qxc6 Rf8 20. Re8 h6 21. Rxf8+ Kxf8 22. Qa8+ Ke7 23. Qe4+ Kf8 24. Be5 Qb6+ 25. Bd4 Qb8 26. Qg4! .

And that got me wondering in the 17. Rxa7 line about whether castling was possible for black. Normally CG.com don't use puzzles where castling is an option as we have no way of knowing whether it is legal or not.

In a traditional puzzle the usual convention is to assume that castling is allowed unless specifically told otherwise. But these aren't traditional puzzles - they are real games - so does that convention also apply here? I can't recall it ever coming up before.

trees falling . . .
bears pooping . . .
unable to perceive the 5th dimension . . .

we humans question what we believe. So, for todays puzzle, I guess I will add one more

White has a bishop and a knight for the bishop pair and a pawn.

The black DSB is the only defender of the black king. Therefore, 17.Rxe7+:

A) 17... Kxe7 18.Qd6+ Ke8 19.Re1+ and mate in two.

B) 17... Kf8 18.Qd6

B.1) 18... Qxa1 19.Rd7+ Kg8 (19... Ke8 20.Qe7#) 20.Rd8+ Be8 21.Rxa8 Qa4 22.Qe7 Qxc2+ 23.Kg3 Qg6+ 24.Ng5 + -.

17. Rxe7+ Kf8
18. Qd6 Kg8
19. Qxc6 Rf8
20. Re8 h6 and white has a lot of work to do.

This one:

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Fritz gives this an eval of +2.7 ish with ideas of Qe5 and Bd5.

He also says that black can survive the earlier variation 18. Rxf7+ Kxf7

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With an eval of -6.8 (ie no white mate and black gets to keep the material)

Or 20. Rxf7+ Kxf7 which he thinks is a draw by perpetual.

I was wondering about 18. Ne5. Lots of promising lines but in all the ones I looked at B has a defence. eg. 18 ... Be8 19 Rxe8+ Rxe8 20 Bd6+ Kg8 21 Qf3 Qxc2+ 22 Kg3 looked good but then ... Qa2.