Course introduction
A guided path from first rules to expert proof
You will learn: how Sudoku School builds one solving habit at a time across 40 lessons.
Sudoku School is the structured path through Sudoku Skills. It starts with board language and clean candidates, then moves through singles, locked candidates, pairs, triples, fish, wings, coloring, chains, and What-If proof.
- Focus cells show the cells that prove the idea.
- Candidate notes show the exact evidence the technique uses.
- Red notes show candidates that the proof removes.
- Board animation lets you play, pause, reset, and step through named techniques on the live board.
C1C2C3C4C5C6C7C8C9R1R2Box 1Box 2Box 3R3R4R4C1R5Box 4Box 5Box 6R6R7R8Box 7Box 8Box 9R9
Course map: The School starts with the board language beginners need: rows, columns, boxes, and cell names. Every later proof uses these same coordinates so the reason for each move is easy to follow.
How the School works
Read the goal, solve the puzzle, review the proof
You will learn: how lessons, coach nudges, progress, badges, and the Diploma fit together.
- Lessons unlock in order. Each new idea depends on the habits you practiced earlier.
- The coach helps without taking over. Reveal nudges only when you need a clearer path.
- Notes become evidence. Later lessons are easier when your candidates stay clean.
- Walkthrough review matters. Completed lessons can be reviewed so the pattern becomes recognizable.
- Badges and the Diploma reward mastery. Full Access membership includes the complete School path.
- 1GoalKnow the habit or technique before touching the board.
- 2TryStudy the givens and candidates, then make the move yourself.
- 3NudgeReveal nudges only when you need a clearer path.
- 4ReviewCheck the proof so the pattern becomes recognizable next time.
What to expect: Sudoku School is hands-on. It teaches one idea, lets you practice it on the board, then reinforces the reason before the next lesson unlocks.
Board reading
Givens and assist colors
You will learn: how fixed clues and visual assists turn the board into usable evidence.
- Givens are locked starting clues. They are facts, not guesses.
- Blue assist shows the selected cell's peers: its row, column, and box.
- Yellow assist helps you ask where a selected value is blocked.
- Red circles warn when a selected value duplicates a peer.
537619598686348317266284195879
Teaching snapshot: R1C1 is selected and already contains 5. Blue shows its row, column, and box. Red circles mark other 5s. Yellow appears only because a numbered cell is selected.
Basic method
Legal candidates
You will learn: how to write only the digits that remain possible.
- Start with the selected cell's row, column, and box.
- Remove every digit already present in those three units.
- The digits left are legal candidates. They are not guesses; they are the current possibility space.
531247619598686348317266284195879
Teaching snapshot: The note grid in R1C3 contains exactly 1, 2, and 4. A legal candidate is not a guess; it is a digit not blocked by the row, column, or box.
Basic method
Naked singles
You will learn: how a cell becomes forced when only one candidate remains.
- A naked single is about one cell.
- All other digits have been ruled out by the row, column, and box.
- Because the cell has one legal candidate left, that candidate must be placed.
5376195986863485317266284195879
Teaching snapshot: R5C5 is not a hidden pattern. The cell itself shows the proof: its only remaining note is 5.
Basic method
Hidden singles
You will learn: how to find the only home for a digit inside one unit.
- A hidden single is about a row, column, or box.
- The target cell may still have several notes.
- If a digit has only one possible home in that unit, it must go there.
5376195129823342413457624786348317266284195879
Teaching snapshot: R3C7 still has several notes, so it is not naked. It is hidden because row 3 has no other 5 candidate.
Intermediate method
Locked candidates
You will learn: how a digit trapped in one place removes candidates somewhere else.
- Pointing: inside one box, all homes for a digit sit in one row or column.
- Claiming: inside one row or column, all homes for a digit sit in one box.
- Either way, the digit is reserved for that overlap, so matching candidates outside the overlap can be removed.
29491959
Teaching snapshot: Only the relevant 9 notes are shown. This is a pointing pattern: box 1 points its 9 into column 2, so the red 9s outside the box must go.
Intermediate method
Pairs
You will learn: how two cells or two digits reserve space for each other.
- Naked pair: two cells in one unit contain exactly the same two candidates.
- Hidden pair: two digits have only the same two homes in one unit.
- Once the pair is reserved, those digits can be removed from places where they no longer belong.
2612624626826
Teaching snapshot: This is a naked pair. The pair cells reserve 2 and 6, so the red 2s and 6s in the rest of row 5 are not possible.
Intermediate method
Triples
You will learn: how the pair idea expands to three cells or three digits.
- A naked triple reserves three digits across three cells in one unit.
- A hidden triple finds three digits whose only homes are the same three cells.
- Triples reward careful notes. One extra candidate can make a false pattern look real.
1571259154578571679
Teaching snapshot: A triple is the same reservation idea as a pair, but with three cells and three digits.
Advanced method
X-Wing
You will learn: how two rows and two columns can lock a digit into a rectangle.
- For one digit, two rows each have exactly two possible homes.
- Those homes line up in the same two columns, forming a rectangle.
- Those columns must use the digit in the rectangle, so outside candidates in those columns can be removed.
173727279475727178
Teaching snapshot: The purple cells form the four corners of the X-Wing. The red 7s share the same columns but are not corner cells, so they are removed.
Advanced method
Swordfish
You will learn: how the X-Wing reservation grows to three rows and three columns.
- A row-oriented Swordfish uses three rows whose candidate positions are confined to the same three columns.
- A column-oriented Swordfish uses three columns whose candidate positions are confined to the same three rows.
- The pattern is the same relationship, just rotated: rows reserve columns, or columns reserve rows.
- Sudoku School includes both orientations, so the pattern is learned as a row-column relationship instead of a memorized shape.
251558354557152515355645
Teaching snapshot: Swordfish is not a new kind of guess. It is the same row-column reservation idea as X-Wing, but with three rows and three columns. Row-oriented and column-oriented Swordfish are the same proof viewed from opposite directions.
Advanced method
XY-Wing
You will learn: how three bivalue cells can prove a candidate removal.
- The pivot has candidates X and Y.
- One wing has X and Z. The other wing has Y and Z.
- Whichever value the pivot takes, one wing becomes Z, so any cell that sees both wings cannot be Z.
37231312
Teaching snapshot: R2C2 sees both wings: R5C2 in its column and R2C5 in its row. Since one wing must be 3, R2C2 cannot be 3.
Advanced method
Coloring
You will learn: how alternating links for one digit reveal traps and contradictions.
- Blue and green marks are opposite possibilities for the same digit.
- In a trap, a target cell sees both colors, so it cannot contain that digit.
- In a same-color conflict, one color contradicts itself, so the opposite color must be true.
244834144547
Teaching snapshot: The red 4 is not part of the colored chain. It sees both possible colors, which means it would conflict no matter which color wins.
Expert method
Forcing chains
You will learn: how a sequence of implications can prove a placement or removal.
- A chain starts with a small assumption and follows only forced consequences.
- If every branch proves the same endpoint, that endpoint is safe.
- If a branch contradicts the puzzle, the starting candidate can be removed or the opposite candidate can be placed.
17142747
Teaching snapshot: Chains are easier when read one link at a time. The target R6C2 sees both possible 7 endpoints, so the red 7 cannot remain.
Expert method
What-If reasoning
You will learn: how to test a small branch and return with proof.
- What-If saves the decision board before testing an assumption.
- The branch then plays forward until it succeeds, gets stuck, or contradicts the puzzle.
- If it fails, the app rewinds to the saved board and explains what the failure proves.
- Expert lessons use What-If to teach disciplined proof, not random guessing.
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Real decision snapshot: The purple cells are the whole branch point. This page is not claiming the branch is solved immediately; it shows the exact board the lesson would save before testing one side and rewinding if needed.
Master the Grid
Sudoku Skills: Master the Grid