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It would be interesting to know how earl eliminated 8 from r9c1. The best move I could find is a Sue-De-Coq on box 1 and column 1. A rare, but beautiful move.

Ruud

Here is another One-Trick Pony.

A rather nice step cracks the puzzle, but there is a less elegant alternative that achieves the same result.
1 9 .|. 5 .|. . 4
. . 7|. . .|3 . .
. . .|9 . 3|. 2 5 ...

There's more info on this puzzle at the [url=http://www.sudoku.com/boards/viewtopic.php?p=37328#37328]Player's Forum.

It is no longer considered the hardest Sudoku ever. Here's a [url=http://www.su ...

This move is called BUG-Lite.

A size 12 Deadly Pattern is very rare.

Ruud

Hint:
.------------------.------------------.------------------.
| 4 8 7 | 3 12 12 | 56 9 56 |
| 59 359 39 | 6 48 48 | 2 7 1 |
| 1 2 ...

This is a very deceiving puzzle.

You need 2 naked pairs and 4 hidden pairs to get to this point:

.------------------.------------------.------------------.
| 4 8 7 | 3 12 12 ...

Here is a puzzle from the long forgotten top95.

4 8 .|3 . .|. . .
. . .|. . .|. 7 1
. 2 .|. . .|. . .
-----+-----+-----
7 . 5|. . .|. 6 .
. . .|2 . .|8 . .
. . .|. . .|. . .
-----+-----+---- ...

There is another UR move, but I'm not sure what type it is. It is the UR equivalent of the Broken Wing + Guardians technique.

Here's the grid:

+----------------+----------------+---------------- ...

Maybe you could ask Dr. Green Mr. Green

and Dr. Ross Embarassed

to help you...

Recovering a lost puzzle is just as much fun as solving one Very Happy

This is your grid in a better format:

+-------------+-------------+-------------+
| . . . | 27 . . | . . 27 | ...

The last xy-chain can also be shortened to this AIC:

(4=5)r6c1 - (5)r1c1 = (5)r1c5 - (5=4)r7c5

The strong link for digit 5 in row 1 replaces the longer route via r1c8.

This no longer qualifi ...

After the basic steps, including an XY-Wing, there are 4 XY-chains with some cleanup inbetween.

(5=8)r1c5 - (8=3)r1c8 - (3=6)r3c8 - (6=5)r2c9
(this grid also allows a Sue-de-Coq move, but that d ...

Alternately...

.---------------.---------------.---------------.
|-13 6 7 | 139 8 5 | 2 4 *39 |
| 123 8 9 | 4 123 12 | 7 6 5 |
| 4 *23 5 | 237 2369 2 ...

Even though it does not really advance the puzzle (you still need the skyscraper to break it), I found a nice BUG-Lite type 3 in this puzzle:

.---------------.----------------.---------------.
| 1 ...

Thanks Mogulmeister for the correction.

I found another one in the recent Andrew Stuart puzzle, but that does not really advance the puzzle. These patterns occur frequently in Sudoku puzzles, but b ...

If you do not fancy unique rectangles (who doesn't?), there is a nice 2SDG (Sue de Coq) alternative:

r56c8 has candidates {4,5,8,9}
because r4c8 = {4,8}, r56c8 must contain either 5 or 9.
because ...

This will work: http://www.ukpuzzle.com/sudokuarchive/524.html

Alternative solution in 2 steps:

2-String-Kite for digit 1 in row 3 and column 5 eliminates digit 1 from r7c8, so you can place digit 8 in r7c8.

XY-chain: (8=7)r1c7 - (7=1)r3c8 - (1=2)r6c8 - (2= ...

...if they were present... they are not.

A finned squirmbag in columns 12578, rows 16789 with a fin in R2C1 would eliminate all candidates in columns 1&2 which can "see" the fin in R2 ...

You have correctly identified a finned squirmbag, but it does not yield any eliminations.

EITHER the candidate 2 in R2C1 is true OR the squirmbag for digit 2 in columns 12578 is true. You would onl ...