Fundamentals
Full House: One gap, one missing digit
A Full House has just one empty cell in a row, column or 3×3 box. The one missing digit belongs in that cell.
Read more: Full House: One gap, one missing digitBack to the learning guidesFundamentals
Sudoku strategy: choosing a technique for the grid
Singles, pairs, wings and chains address different situations. Find their prerequisites and a guide to the pattern you need.
Choose a technique whose prerequisites make sense to you. A complex name doesn’t improve a simple move. After placing a number or removing a candidate, check for singles again.
When do you need pencil marks?
Full House needs only an area with one gap. Naked Single and Hidden Single apply the basic rules to the remaining possibilities. Pairs, wings and chains need an accurate candidate state in the areas being examined.
Understand one move
Each introduction explains its core idea using one example. Use the groups below to find a guide; the app covers further variations and guided practice.
Keep practicing in Unisu
Download Unisu for iPhone or Android. Solve a puzzle or work through a guided lesson on the technique.
Fundamentals
Fundamentals
Naked Single: One candidate left in a cell
A Naked Single is a cell with exactly one candidate remaining. Its row, column and box rule out every other digit.
Read more: Naked Single: One candidate left in a cell
Fundamentals
Hidden Single: One place left for a digit
A Hidden Single occurs when a digit has only one possible place in a row, column or box. The cell itself can still have other candidates.
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Core techniques
Core techniques
Locked Candidates: Use the overlap between a box and a line
Locked Candidates: when all candidates for a digit in a box lie on one row or column, remove that digit from the rest of that line outside the box.
Read more: Locked Candidates: Use the overlap between a box and a line
Core techniques
Naked Pair: Two cells reserve two digits
A Naked Pair has two cells in one row, column or box with only the same two candidates. Remove those digits from the other cells in that area.
Read more: Naked Pair: Two cells reserve two digits
Core techniques
Hidden Pair: Two digits have just two places
If two digits can appear only in the same two cells of an area, they form a Hidden Pair. Other candidates can be removed from those two cells.
Read more: Hidden Pair: Two digits have just two places
Core techniques
Naked Triple: Three cells share three candidates
A Naked Triple has three cells in one area with just three different candidates between them. Remove those digits from the other cells in that area.
Read more: Naked Triple: Three cells share three candidates
Core techniques
Hidden Triple: Three digits confined to three cells
A Hidden Triple occurs when three digits have only three possible cells between them in a row, column or box. Extra candidates in those cells can be removed.
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Further techniques
Further techniques
X-Wing: Two rows, the same two columns
An X-Wing has a digit in exactly two possible cells in each of two rows, in the same two columns. Learn which candidates this pattern rules out.
Read more: X-Wing: Two rows, the same two columns
Further techniques
XY-Wing: A pivot and two wings
An XY-Wing connects a pivot cell X/Y to two cells it sees, X/Z and Y/Z. Whichever digit goes in the pivot, one wing must contain Z.
Read more: XY-Wing: A pivot and two wings
Further techniques
XYZ-Wing: Three candidates in the pivot
An XYZ-Wing has a pivot X/Y/Z and two cells it sees, X/Z and Y/Z. At least one of these three cells must contain Z.
Read more: XYZ-Wing: Three candidates in the pivot
Further techniques
Swordfish: Follow one digit across three lines
A Swordfish confines a digit’s candidates in three starting rows to exactly three columns. The three required placements use those columns.
Read more: Swordfish: Follow one digit across three lines
Further techniques
W-Wing: A matching pair joined by a strong link
A W-Wing links two cells with the same candidate pair through a strong link. Follow the example to see how that connection rules out another candidate.
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Advanced techniques
Advanced techniques
Simple Coloring: Follow one digit in two groups
Simple Coloring follows strong links for one digit. When an area has exactly two places for it, one must be true. Alternating groups show those dependencies.
Read more: Simple Coloring: Follow one digit in two groups
Advanced techniques
Jellyfish: Four lines form one pattern
A Jellyfish confines a digit’s candidates in four rows to four columns. Learn how this pattern rules out candidates outside those starting rows.
Read more: Jellyfish: Four lines form one pattern
Advanced techniques
Unique Rectangle: Avoid a second solution
Unique Rectangle requires a guaranteed unique solution. This example uses four cells in two boxes to show how that condition rules out a candidate.
Read more: Unique Rectangle: Avoid a second solution
Advanced techniques
Finned X-Wing: An X-Wing with an extra possibility
A Finned X-Wing adds a fin to the basic pattern. Remove only candidates that conflict with both the fin and the X-Wing left when the fin is false.
Read more: Finned X-Wing: An X-Wing with an extra possibility
Advanced techniques
XY-Chain: Link cells with two candidates
An XY-Chain links cells with exactly two candidates each. Follow the example to see where the shared digit at the two ends can be eliminated.
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Master class
Master class
AIC: Alternate strong and weak links
An AIC connects candidates using alternating strong and weak relationships. An assumption can then be followed along the chain to a certain deduction.
Read more: AIC: Alternate strong and weak links
Master class
ALS: Connect almost locked candidate groups
An ALS is a group of n cells in one area with n+1 distinct digits among their candidates. Two such groups can interact through a restricted common candidate.
Read more: ALS: Connect almost locked candidate groups
Master class
Forcing Chain: Examine two possibilities separately
A Forcing Chain checks both candidates of a cell in separate branches. If both branches force the same elimination, you can apply it with certainty.
Read more: Forcing Chain: Examine two possibilities separately
Master class
Nishio: Test an assumption through contradiction
Nishio tests one candidate by following the moves it forces. If it leaves a cell with no option or a required digit with no place, reject that candidate.
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Where to go next
Fundamentals
Sudoku pencil marks: track and update candidates
A small note records a possibility. Learn how candidates differ from placements and how to keep them up to date.
Read more: Sudoku pencil marks: track and update candidates
Fundamentals
Solve Sudoku without guessing: a repeatable search order
Check nearly full areas, then candidates and singles. Build a routine for finding a move you can explain.
Read more: Solve Sudoku without guessing: a repeatable search order
Fundamentals
Sudoku difficulty levels: what makes a puzzle demanding?
The techniques you need tell you more than the number of gaps. See how Unisu organizes its 16 game levels.
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Frequently asked questions
Does one technique solve every Sudoku immediately?
No. Each method needs particular candidate relationships. Its name does not justify a move when those conditions are absent.
In what order should I look for techniques?
Check missing digits and Singles first. Move to smaller candidate patterns before following long chains.
Why do complete candidates matter?
A missing possibility can create a false pattern. Work from the actual candidates, rather than assuming the displayed notes are complete.
How do pairs, Fish and Wings differ?
Pairs reserve digits within an area. Fish connect one digit’s positions across rows and columns; Wings use relationships between cells with few candidates.
Where can I practice the methods?
Unisu has guided lessons on its active techniques. Choose a method whose prerequisites you know before attempting longer chains.
Ready for a puzzle? Take a moment.
Download Unisu on your phone. Start a Sudoku or choose a lesson on the technique you want to understand.
