Sudoku-YAdvanced
X-Wing: four corners with a reason behind them
X-Wing tracks one digit across two rows or columns. A rectangle alone is not enough: the candidate positions in the starting units must be restricted.
Practise on a board ↓Interactive practice · Your saved game is unchanged
Prove an X-Wing
In rows B and H, 6 can appear only in columns 2 and 8. Compare the two cases, then click 6 at C2 and F8 to eliminate it.
An original partial candidate scenario. Only relevant candidate 6s are shown; dotted cells do not mean no candidates. Rows B and H have exactly the two positions for 6 shown here.
Progress: 0 / 2
Show reasoning and answer
If B2 = 6, H8 must be 6; if B8 = 6, H2 must be 6. Both cases use 6 in columns 2 and 8, so C2 and F8 cannot contain 6. Their final values are still unknown.
LOOK · TRY · UNDERSTAND
Try it and see why
Four corners are not enough
Rows B and H have no placed 6. Every possible spot for that digit in those rows is shown. Only the digit we are tracking is displayed here. A lone small digit is not a Single.
Our own partial teaching board, not a full puzzle. Large digits are placed values; small digits are candidates. Dots mean the cell is not described here. Complete candidate lists are part of the stated scenario, not calculated from the dots.
Green outline = evidence · Purple “try” = an assumption · Crossed out = removed candidate
Can the four-corner X-Wing remove 6 at C2?
See the reasoning
Either B2/H8 or B8/H2 uses both columns 2 and 8, so C2 cannot be 6.
Digit set 1/9 · A different digit set in the same pattern, not a new full puzzle.
Read the source principles: HoDoKu — Basic Fish
Check the condition, not just the shape
In a new example, row B allows 6 at B2, B5 and B8, while row H allows 6 at H2 and H8. Can a row-based X-Wing in B/H still eliminate 6 from C2?
Check the reasoning
No. Row B has a third position. With B5 = 6 and H8 = 6, those rows do not reserve column 2, and C2 = 6 conflicts with neither placement. The original four corners no longer justify removing 6 from C2.
On your game: follow one digit through all positions in two source rows. Each must allow it in exactly the same two columns. Eliminate in those columns outside the source rows, keeping the four corners.
The row-based condition
Choose a digit still missing from two rows. Each row must have exactly two possible positions for it, using the same two columns. You can then eliminate the digit from other cells in those columns.
SudokuWiki — X-Wing Strategy · HoDoKu — Basic Fish
| Row | Column 2 | Column 8 |
|---|---|---|
| B | B2: 5 ? | B8: 5 ? |
| C / F | C2: | F8: |
| H | H2: 5 ? | H8: 5 ? |
B and H have no other positions for 5. C2 and F8 are sample elimination targets.
Four corners for digit 5
Suppose row B allows 5 only in B2/B8 and row H only in H2/H8. If B2 is 5, H8 must be 5; if B8 is 5, H2 must be 5. Either way, columns 2 and 8 receive their 5s from these two rows. Eliminate 5 at C2 and F8.
Try before revealing
Can the same row-based X-Wing be used if B5 also allows 5?
Show explanation
No. Row B is no longer restricted to columns 2 and 8. Check all positions in the two source rows, not just the four corners.
Keep sources and targets separate
Starting from rows gives eliminations in columns outside those rows. Do not remove the four corner candidates. A column-based X-Wing reverses the roles. Swordfish extends the basic fish idea to three source units with additional conditions.
In Sudoku-Y
The Techniques scanner now detects row- and column-based X-Wing on your current board. Select a result to see its corners, elimination targets and reasoning, with a link to this lesson. It does not alter notes or fill answers for you.
References and further reading
Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.