Sudoku-Y

SUDOKU-Y / FIELD NOTES

Sudoku techniques and learning library

Understand the move before entering the number. 39 practical guides connect the rules, candidate eliminations and harder reasoning, with original examples and links to their sources.

New here? Start with the rules →

Try the 12-topic practice lab ↓

7 GUIDES

Start here and get unstuck

Rules, notes and a route to your next move

  1. 01
    Foundations

    Pencil notes: candidates are not confirmed answers

    Learn to separate notes from answers, remove candidates using rows, columns and boxes, and keep clear notes in Sudoku-Y.

    Practise on a board →
  2. 02
    Foundations

    Naked Single vs Hidden Single: cell or digit?

    Understand the difference between one candidate in a cell and one position for a digit, with a worked example and a quick question.

    Practise on a board →
  3. 03
    Understanding the game

    What makes Sudoku harder than Expert?

    Understand technique-based difficulty, solving paths and clue counts, including what Sudoku-Y’s current difficulty labels actually mean.

  4. 04
    Foundations

    Scanning Sudoku: follow one digit to find a first move

    Practise scanning one digit through a box with an original partial example and learn why notes cannot replace placed clues.

  5. 05
    Foundations

    When a mistake spreads: trace the reason, not just the duplicate

    Diagnose duplicate values, empty candidate sets and lost candidates without treating an assumption as a confirmed move.

  6. 06
    Foundations

    Stuck in Sudoku? A short route before guessing

    A practical checklist for incomplete notes, singles, pairs and hints, with a worked hidden-single example.

  7. 07
    Foundations

    A small Sudoku practice routine: understand one move

    A flexible practice plan from Sudoku-Y, using explanations and changed conditions rather than memorising answers.

11 GUIDES

Build your technique toolkit

From candidate pairs to fish and short chains

  1. 08
    Next steps

    Naked Pairs: two cells reserve two digits

    Learn exactly where a naked pair eliminates candidates using an original {2,8} example, plus a counterexample to test your reasoning.

    Practise on a board →
  2. 09
    Next steps

    Locked Candidates: pointing versus claiming

    Learn the direction of pointing and claiming, when to inspect a whole box or row, and where digit 6 can be eliminated.

    Practise on a board →
  3. 10
    Advanced

    X-Wing: four corners with a reason behind them

    Work through a row-based X-Wing using digit 5, check its exact conditions and eliminate candidates from the correct columns.

    Practise on a board →
  4. 11
    Next steps

    Hidden pairs and triples: digits reserve cells

    Find Hidden Pairs and Hidden Triples by checking every position for selected digits, then distinguish kept candidates from eliminations.

  5. 12
    Next steps

    Naked triples and quads: count the candidate union

    Learn why Naked Triples and Quads need not have identical notes, with worked candidate sets and a counterexample.

    Practise on a board →
  6. 13
    Advanced

    Swordfish: one digit across three rows

    Check a basic Swordfish using three rows and three columns, recognize extra positions and eliminate only outside the sources.

    Practise on a board →
  7. 14
    Advanced

    XY-Wing: two pivot cases, one shared elimination

    Follow an XY-Wing or Y-Wing through its pivot, two wings and common target, without a long speculative solving sequence.

    Practise on a board →
  8. 15
    Advanced

    When to assume—and when a branch is actually disproved

    Understand why no conflict is not proof, how contradictions eliminate assumptions, and how to use independent what-if cases in Sudoku-Y.

    Practise on a board →
  9. 16
    Next steps

    Strong and weak links: what follows if this is false?

    Understand strong and weak Sudoku links through two positions, an added third option and an original reasoning challenge.

  10. 17
    Advanced

    Skyscraper: two pairs, a useful shared consequence

    Follow an original Skyscraper diagram through both cases, check common targets and see why a third candidate breaks the argument.

  11. 18
    Advanced

    Two-String Kite: connect a row and column through a box

    Learn a Two-String Kite with an original diagram, two short conditional paths and a counterexample when an extra position appears.

12 GUIDES

Explore advanced reasoning

Read the examples, then use Techniques in Classic 9×9 to find applicable moves

  1. 19
    Advanced

    XYZ-Wing: the target must see all three cells

    Extend XY-Wing with a three-candidate pivot, follow all three cases, and check a target that sees only the wings.

  2. 20
    Advanced

    W-Wing: matching pairs joined by a bridge

    Join two matching bivalue cells through a strong link and prove an elimination from their common peers.

  3. 21
    Advanced

    Remote Pairs: pass a pair along a chain

    Follow identical bivalue cells, count the links, and see when opposite endpoints remove both digits from a common peer.

  4. 22
    Advanced

    Simple Coloring: two colors, one connected argument

    Color strong links for one digit, follow a color trap, and avoid mixing colors from unrelated components.

  5. 23
    Advanced

    X-Chains: follow one digit between two useful ends

    Read alternating strong and weak links with a six-node implication example and a deliberately broken connection.

  6. 24
    Advanced

    XY-Chains: change digits, then meet the same digit again

    Follow four bivalue cells to eliminate 2, and see exactly where an extra candidate breaks the implication.

  7. 25
    Advanced

    Jellyfish: four rows reserve four columns

    Extend the basic fish argument to four units, with original candidate-6 positions and a fifth-column warning.

  8. 26
    Advanced

    Finned X-Wing: an extra candidate narrows the targets

    Split the fin into true and false cases, then choose a target blocked both by the fin and by the underlying X-Wing.

  9. 27
    Advanced

    Empty Rectangle: inspect the missing candidate positions

    Combine a box restriction with an outside conjugate pair, then test an extra candidate that breaks the elimination.

  10. 28
    Advanced

    Unique Rectangle: uniqueness is an extra premise

    Understand Type 1 with a two-box example, and check the unique-solution assumption, givens and variant restrictions.

  11. 29
    Advanced

    BUG+1: one triple-candidate cell is not enough

    Check the whole candidate structure before choosing the extra digit, with a conditional example and a failed unit-count check.

  12. 30
    Advanced

    Almost Locked Sets: one digit short of a locked set

    Count cells and distinct candidates, inspect a two-cell ALS, and see why outside eliminations still need another condition.

5 GUIDES

Killer: cages and sums

Separate combinations, positions and counted areas

  1. 31
    Foundations

    Killer Sudoku: cages and sums

    Learn cage totals, distinct digits, combinations and the rule of 45, with an example that separates a digit combination from a confirmed placement.

    Practise on a board →
  2. 32
    Foundations

    The rule of 45: count the cells before subtracting

    Use unit totals in Killer Sudoku, spot cages that cross the boundary, and adjust the total for 4×4, 6×6 and 16×16 boards.

  3. 33
    Foundations

    Killer combinations: a set is not an order

    List distinct digits for a cage total, filter combinations against candidates, and separate a known set from confirmed cell values.

  4. 34
    Next steps

    Innies and outies: missing cells and extra cells

    Subtract unit and cage totals to find inside or outside cells, without double-counting or inventing a new no-repeat cage.

  5. 35
    Next steps

    Cage intersections: let a required digit block a row

    Connect Killer combinations with row and box restrictions, and separate a digit present in every combination from one merely allowed.

4 GUIDES

Board sizes and other rules

Different sizes are playable; Diagonal, Jigsaw and Even/Odd are reading guides

  1. 36
    Foundations

    Sudoku Sizes Compared: 4×4, 6×6, 9×9 & 16×16

    Compare Sudoku sizes with clear grid pictures: numbers, box shapes and which board to try first. Play 4×4, 6×6, classic 9×9 or 16×16 for free.

  2. 37
    Understanding the game

    Diagonal Sudoku: two more no-repeat lines

    Meet Sudoku X, follow a diagonal single, and see why the central cell belongs to two extra units.

  3. 38
    Understanding the game

    Jigsaw Sudoku: follow the region boundary

    Read irregular nine-cell regions instead of fixed 3×3 boxes, with an original coordinate example and a boundary check.

  4. 39
    Understanding the game

    Even/Odd Sudoku: parity is another candidate filter

    Use even and odd markers to narrow candidates, and learn why an unmarked cell does not automatically mean the opposite.

LOOK · TRY · UNDERSTAND

Try it and see why

Where can a digit repeat?

A1 has a placed 5. Check its row, column and box.

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.

5
·
·
·
5
·
·
·
·
·
5
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
5
·
·
·
5
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·
·

Green outline = evidence · Purple “try” = an assumption · Crossed out = removed candidate

Which cell does not conflict with the 5 at A1? Check only the shown clue; this does not prove the cell’s answer.

See the reasoning

E5 shares none of A1’s three units. This 5 does not block it, but other clues still need checking.

Digit set 1/9 · A different digit set in the same pattern, not a new full puzzle.

Read the source principles: Nikoli — Sudoku