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A little practice. A satisfying break.

How to play Sudoku

Nine digits, three kinds of checks. Learn to turn a possible number into a reasoned next move.

Play Sudoku

Already playing in another tab? Switch back to that tab to keep your board and Undo history. These examples are separate from your puzzle.

The rules: rows, columns and boxes

Sudoku is a 9 × 9 grid split into nine bold 3 × 3 boxes. Fill each empty cell with a digit from 1 to 9 so that every row, every column and every box contains each digit exactly once.

The bold starting clues stay fixed. You do not add the numbers or make sums: their identities and positions are what matter. A number can repeat elsewhere on the board, but never in the same row, column or box.

One box, enlarged. A full board has nine such boxes. Its rows and columns must also obey the no-repeat rule.

Before writing a number, check all three groups containing its cell. A number that fits the row may still conflict with a clue above it or inside its box.

Two worked examples

1. Find the last missing number

Before

Row fragment: the fifth cell is the only gap. Its column and box also allow 5.

Action: Enter 5 in row 1, column 5.

After

The row is complete. The new 5 also fits its column and box.

The row already contains 1, 2, 3, 4, 6, 7, 8 and 9. Of the allowed digits, only 5 is missing. Since the column and box allow 5 too, put 5 in the gap. If they did not, you would need to recheck an earlier entry, not break a rule.

2. Use a pair to unlock a third cell

A, B and C are the row’s only empty cells. These are complete candidates after checking rows, columns and boxes, not partial player notes.

Call the cells A, B and C from left to right. A and B each allow only 2 or 7. They cannot both be 2, and cannot both be 7, because they share a row. Whichever order they take, they use both 2 and 7. Neither number can go in C, leaving 5.

This is a naked pair. You have proved C without choosing whether A is 2 or 7. Two cells with the same two pencil marks are not enough if those marks omit another legal candidate.

Walkthrough: Four singles, three kinds of check

An original near-complete 9 × 9 teaching puzzle. Start with a column, then revisit the row and box. This small lesson is separate from your saved games.

Use fullscreen for a larger board, or follow the text lesson below.

Before

Original starting clues; question marks are the cells to solve.

Text transcript & checked moves

  1. Column first. Column 1 already contains 2 through 9. Its only gap, row 1 column 1, must be 1. That 1 also fits its row and top-left box.
    Show board after move 1
    Place 1 at row 1, column 1.
  2. Return to the row. Row 1 now has every digit except 2. Enter 2 at row 1 column 2; its column and box also allow 2.
    Show board after move 2
    Place 2 at row 1, column 2.
  3. Finish the box. The top-left 3 × 3 box now lacks only 5. Enter 5 at row 2 column 2. Its row and column have no other 5.
    Show board after move 3
    Place 5 at row 2, column 2.
  4. Check the centre. Row 5 has one gap and lacks 9. Enter 9 at row 5 column 5. Column 5 and the centre box also lack 9.
    Show board after move 4
    Place 9 at row 5, column 5.

Every row, column and 3 × 3 box now contains 1–9 once each. A useful loop: place one proved number, then rescan the groups it changed. Try your own Sudoku next.

Try your own Sudoku

Build from singles to pairs

  1. Scan nearly full groups. A row, column or box with one gap is a useful starting point. After placing a number, check the groups it affects.
  2. Look for a naked single. List the digits not already used in the cell’s row, column or box. One remaining possibility gives the answer.
  3. Look for a hidden single. Choose a missing digit in a group and check all its empty cells. If just one cell can take it, place it there, even if that cell has other candidates.
  4. Follow locked candidates. If every possible place for 6 in a box lies on one row, 6 cannot appear outside that box on the same row. Conversely, if all places for 6 in a row lie in one box, remove 6 from the rest of that box. The same reasoning works with columns.
  5. Find naked pairs. Two cells in one row, column or box whose complete candidates are the same two digits reserve them. Remove those digits from the other cells in that shared group.
  6. Find hidden pairs. If two missing digits can appear only in the same two cells of a group, those cells must contain them. Remove other candidates from those two cells. Check every cell in the group before concluding that the pair is hidden.

Common mistakes

Do not treat a plausible number as a forced one, scan only the row, or assume a blank note list means no candidates. Pair deductions apply only to a shared row, column or box; matching marks in unrelated cells prove nothing. If the board stops making sense, recheck entries and use Undo before adding more guesses.

Learn the rules behind a hint

Start with singles. The later lessons explain more involved deductions when you need them. A candidate is a number that is still possible in a cell after checking the rules and earlier deductions. Your pencil marks may be incomplete: a missing note is never proof that a number is impossible.

The last missing number

A row, column or box needs each number from 1 to 9 exactly once. If eight cells contain 1, 2, 3, 4, 6, 7, 8 and 9, its last cell must be 5. The cell must also obey its other two groups; a conflict means an earlier entry needs rechecking.

Only one possible number

Check a cell’s row, column and box, plus any possibilities already ruled out in earlier checked steps. If they rule out every number except 3, the cell must be 3. For example, a cell that allowed 3 or 7 becomes a single only after a preceding deduction rules out 7. That conclusion does not follow from the visible entries alone.

Only one place in a group

Choose one missing number and check every empty cell in its row, column or box. If 3 can go in only one of them, put 3 there. That cell can have other possibilities: it wins because the group needs its 3. Earlier deductions can remove the alternative locations; read those steps first when the hint says “After ruling out…”.

A number locked to a line

If every possible place for 6 in one box lies in the same row, that row gets its 6 inside the box. Rule out 6 in the rest of the row, outside that box. The same works for a column. Check all places for 6 in the box, not just the notes you happened to write.

A number locked to a box

If every possible place for 6 in one row lies inside the same box, the row puts its 6 in that box. Rule out 6 in the other cells of the box, outside the row. The same works for a column. This reverses pointing: the row or column supplies the complete set of source locations.

Cells reserving numbers

Two, three or four cells in one shared row, column or box reserve the same number of possibilities between them. For example, three cells allowing {1, 2}, {2, 7} and {1, 7} use up 1, 2 and 7 in that group. Other cells in that shared group cannot contain any of those numbers.

Each cell need not allow every reserved number, but the combined set must have exactly as many numbers as cells. Matching notes in unrelated cells do not establish this rule. A pair is the two-cell version; a quad uses four cells and four numbers.

Numbers reserving cells

Check all places for two, three or four missing numbers within one row, column or box. If those numbers can occur only in the same number of cells, those cells are reserved for them. For example, if 2 and 7 have only A and B as possible places in a box, remove other possibilities such as 5 from A and B.

Triples and quads use three and four numbers respectively. Each reserved number needs at least one place. A “direct” version of pointing, claiming or a hidden subset first makes this same exclusion, then finds a single. The hint shows both deductions separately.

Two crossing lines (X-Wing)

Only the four intersections are shown. In rows 2 and 5, every other cell has already been ruled out for 6.

Rows 2 and 5 each need a 6. If their only places for 6 are columns 3 and 8, the two rows must use those two columns once each. Rule out 6 from other rows in columns 3 and 8. This two-row pattern is an X-Wing.

Three crossing lines (Swordfish)

Three rows each need the same number. If all its possible places in those rows lie in exactly three columns, the rows reserve those columns for that number. Rule it out in the other rows of those columns. This three-row pattern is a Swordfish: some intersections may be impossible, but every possible place in the selected rows must be included. Rows and columns can be exchanged.

Four crossing lines (Jellyfish)

Four rows each need the same number. If all its possible places in those rows lie in exactly four columns, the rows reserve those columns for that number. Rule it out in the other rows of those columns. This four-row pattern is a Jellyfish: not every intersection must be possible, but no possible place in a selected row may lie outside the four columns. Rows and columns can be exchanged.

Two choices, one consequence (XY-Wing)

A pivot allows only 1 or 2. One wing allows only 1 or 3, the other only 2 or 3. The pivot shares a row, column or box with each wing. If the pivot is 1, the first wing is 3; if it is 2, the second wing is 3. Either way, at least one wing is 3.

Rule out 3 only from a target that shares a row, column or box with both wings. It need not share a group with the pivot, and the wings need not share a group with each other. For example, pivot at Row 1 Column 1, wings at Row 1 Column 4 and Row 4 Column 1, and target at Row 4 Column 4 satisfy the row/column links.

Three choices, one consequence (XYZ-Wing)

The pivot allows 1, 2 or 3; the wings allow 1 or 3 and 2 or 3. Each wing shares a row, column or box with the pivot. If the pivot is 1 or 2, a wing must be 3. If the pivot is 3, it supplies the 3 itself.

Rule out 3 only from a target sharing a row, column or box with all three source cells. Seeing only the wings is insufficient here. For example, pivot Row 1 Column 1, wings Row 1 Column 4 and Row 2 Column 2, and target Row 1 Column 2 provide the required row and box links.

Notes, hints and controls

On a phone or with a mouse

Cell first is the default: select an empty cell, then tap or click a number on the pad. Turn Notes on to toggle small pencil marks instead of answers. Erase clears the selected cell. Fixed clues cannot be edited.

Number first lets you select a pad digit, then activate each target cell to enter it. With Notes on, each activation toggles that note in an empty cell. Selecting the digit does not write into the previously selected cell. Undo and Redo work one cell edit at a time. Focus, arrow navigation and scrolling never enter a number.

In Sudoku, turn Highlight same number on. In both games, touch-hold a pad digit to lock its highlight; hold it again to unlock, or hold another to switch. Unlock highlight is the visible alternative. A mouse or pen hold highlights only while held. Holds never write. A normal tap on a locked digit still enters it in Cell first or selects it in Number first, keeping the highlight lock.

With a keyboard

Tab to the board; arrow keys move between cells. With a board cell focused, digits enter directly in Cell first. In Number first, a short keyboard digit press selects that number, just like a short pad press; no hold is needed and the selected cell is unchanged. Enter or Space activates the focused cell with that number, or toggles its note with Notes on. N toggles Notes, and Delete, Backspace or 0 erases. Merely focusing a cell does not write. Activating an entry-mode button or a pad digit returns focus to the board after release. Tab leaves the board normally.

Ctrl+Z (Command+Z on Mac) undoes; Ctrl+Shift+Z (Command+Shift+Z) redoes. Ctrl+Y also redoes. These shortcuts work throughout the active play workspace, including hint and action buttons; text fields keep native editing. Holding a digit key does not repeat the edit.

With a board cell focused, Shift+digit locks or unlocks matching displayed numbers and notes. Space enters the locked number, or toggles that candidate in Notes mode, once per press. This also works in Number first: the inspection lock takes precedence for Space; Enter still uses the entry digit. Givens, region limits and assistance rules still apply. Use Unlock highlight to return to matching the selected cell.

With a board cell focused, Shift+digit toggles a highlight lock when Highlight same number is on. Highlighting is inspection, not entry. Dashed pad borders mean nine or more displayed copies, including possible mistakes; those digits are blocked on the pad and for Number first selection, even for notes. In Cell first, Auto-check and challenge mode also block extra keyboard answers; keyboard notes still toggle on empty cells. In Cell first Relaxed play, switching Auto-check off permits additional keyboard answers even when nine copies are displayed.

Use notes as possibilities, not proof

Highlight matching notes in Settings is on by default. It emphasizes individual pencil marks matching the selected, held or locked number, with Notes on or off. Turn it off to highlight only filled numbers. It needs Highlight same number on; turning that master off keeps your separate note choice. Hint emphasis temporarily takes priority.

Write every number that still fits a cell, then revisit the marks after each placement. Missing a pencil mark does not make a number impossible. Notes toggle only in empty cells; an answer can hide earlier notes. Undo restores the previous edit. Automatic note cleanup uses certified-correct placements, so an incorrect answer is not a sound reason to eliminate candidates.

Ask for a reason, then choose whether to apply it

One logical step offers a checked removal of pencil marks actually present, or a proved number with the reasoning needed to place it. Next number continues checked reasoning toward a placement. With no relevant notes to remove, a hint can still find a placement; you do not need to fill every cell with notes first.

The selected deduction’s action and short reason appear as bullets straight away. Use Previous and Next to follow earlier deductions one at a time. The markers follow the selected step; Return to final move restores the final explanation. Full checked evidence & cell links keeps every source, excluded number and Row / Column link available. Learn this rule opens the matching lesson in a new tab, leaving your original puzzle and Undo history in place. For a proved number, Apply enters the answer once, even with Notes on; Undo reverses that edit. For a note-removal hint, Apply note removals removes every listed mark as one undoable change. Candidate exclusions explain a proof and do not ask you to remove absent notes. An incorrect entry is rechecked first. Editing the board or canceling makes an old response unusable. Hints have bounded work limits: a stalled or exhausted result is not a claim that no logical move exists.

Relaxed play and the three-mistake challenge have different assistance rules. A paused or ended attempt protects the board from edits; follow the controls shown for that state. Undo restores entries and notes, but does not refund a challenge mistake or rewind its clock.

Choose a useful difficulty

Start with Very easy or Easy to practice scanning and singles. Medium introduces repeated locked-candidate reasoning. Hard calls for pairs or triples supported by further reasoning; Very hard needs fish, larger subsets or wing deductions alongside sustained supporting work. Fewer starting clues alone do not determine the rating.

Jack’s Break uses five levels based on a checked logical solve and its effort. Open Help → About this difficulty for the actual board’s rating and recorded techniques. These are provisional comparisons on this site’s scale, not player-calibrated ratings or equivalents to another publisher’s labels. A human may find a different route.

Practical questions

Can I replay or share the exact same puzzle?

Yes. Share contains the puzzle code and link. Open a code to return to that identity; opening an existing supported code keeps its original board, even when new-puzzle offerings or difficulty grading have changed. A shared code or link contains the puzzle identity, not your entries or notes. Daily puzzles use the date’s published identity; keep the code if you want to return to that board later.

Are generated puzzles checked for a unique solution?

New puzzles must pass the supported family’s admission checks, including a unique solution and its logical rating requirements, before publication. Generation can fail within its work budget; the app reports that instead of publishing an unchecked board. A rating describes the checked solver’s work, not every human solving path or a promise that any in-progress hint request will finish.

Where is my progress saved?

Entries, notes and play state save locally in this browser after edits. There is no account or cloud backup. Your boards on the home page lists saved puzzles. Private browsing or clearing browser data can remove them. Undo/Redo history stays in the live play page and is not restored by a reload. Guides opened from play use another tab: return to the original play tab to keep that history.

What if saving fails or another tab has changed the save?

Keep the play tab open and follow its recovery message. A changed or unreadable saved copy is not silently overwritten. Use Share → Download this board to keep your current copy; recovery may also offer Download previous copy. Resolve the displayed conflict before relying on a later reload.

How do downloads and restoring work?

A downloaded JSON board includes progress; treat it as your backup rather than a spoiler-free share link. Choose Restore a downloaded board, then review and confirm the replacement. It replaces that board’s entries, notes and local time/challenge state, leaving other boards alone. Undo can restore entries and notes in the live tab, but does not restore time or refund mistakes.

Can I print a puzzle without my answers?

Choose Print board beside the play support controls. The reviewable preview starts with original clues only. Include current entries and Include notes are separate, optional choices; notes appear only in empty printed cells. The sheet includes the exact code and a canonical jacksbreak.com puzzle link, never a solution. Opening or closing the preview does not edit your board; a running timer continues normally.

Choose Print / Save as PDF, then A4 or Letter, portrait, 100% scale, with browser headers and footers off. On a phone, scroll the preview sheet to inspect it. Use Close preview (or Escape) to return to play; canceling the browser’s print dialog also leaves your puzzle unchanged.