Learn / Short chains
Sudoku technique · Short chains · first needed in Tricky puzzles
Empty rectangle
Inside a box a digit lies on two crossing arms. An outside strong link connects one arm to a deduction that attacks the other.
How to spot it
Look for an L-shaped candidate distribution and an outside two-location line.
The common mistake
The intersection in this lesson has no candidate of the focus digit. The shape alone does nothing without the outside link.
Learn this first
Kite or Turbot fish.
A worked example
This position comes from SudokuByte’s lesson on the Empty rectangle. Gold cells hold the evidence; green cells are the pattern’s ends.
2459
3
27
6
1
2479
57
257
8
25689
157
1267
25789
2578
279
4
12567
3
24568
1457
1267
24578
24578
3
1567
9
126
1
9
4
27
27
6
3
8
5
3
6
8
14579
457
479
2
147
19
7
2
5
149
3
8
16
146
169
246
147
9
2478
24678
5
168
3
126
2456
8
126
3
246
24
9
1256
7
256
57
2367
278
9
1
568
256
4
2459
3
27
6
1
2479
57
257
8
25689
157
1267
25789
2578
279
4
12567
3
24568
1457
1267
24578
24578
3
1567
9
126
1
9
4
27
27
6
3
8
5
3
6
8
14579
457
479
2
147
19
7
2
5
149
3
8
16
146
169
246
147
9
2478
24678
5
168
3
126
2456
8
126
3
246
24
9
1256
7
256
57
2367
278
9
1
568
256
4
Where to look
Look for an L-shaped candidate distribution and an outside two-location line.Inside a box a digit lies on two crossing arms. An outside strong link connects one arm to a deduction that attacks the other.
Read the evidence in this position
Inside box 3, 1 is confined to the two arms r2c8 and r3c7, r3c9. The outside strong link is r8c8, r8c3 in row 8.Why the deduction must follow
If r8c8 is 1, it removes 1 from the first arm; the box must put 1 in the second arm. Otherwise the outside strong link forces r8c3 to be 1. A target seeing the whole second arm and that second outside endpoint loses 1.Apply the result
Erase 1 from r3c3. The pattern may remove candidates without immediately solving a cell.
Further reading: hodoku.sourceforge.net