Skudo
ENVI
Play on web

Sudoku techniques, one at a time

A free Sudoku course: every technique the Skudo hint engine knows, from the three rules to 3D Medusa, each with worked examples and solver-checked practice puzzles.

37 of 37 articles published. The rest arrive batch by batch; nothing here links to a page that does not exist yet.

Foundations

  1. Sudoku Rules — the 3 constraintsThe three rules, how to read and name the grid, pencil marks, and your first logical move.
  2. Scanning, Cross-hatching & Last DigitFind digits by eye — cross-hatch a box, fill the last digit, and sweep on, no pencil marks needed.
  3. Killer Sudoku & the Rule of 45The rules of Killer Sudoku, the first easy cage deductions, and the rule of 45 that turns a box's cages into a digit.

Singles

  1. Naked SingleOne cell, one surviving candidate — the safest placement on the grid.
  2. Hidden SingleA digit with only one place left in a row, column or box — even if that cell lists other candidates.

Subsets & Locked Candidates

  1. Locked Candidates — Pointing & ClaimingThe first elimination move: where a box meets a line, pointing and claiming clear a digit until the next single appears.
  2. Naked PairTwo cells locked to the same two digits clear those digits from every unit they share.
  3. Hidden PairTwo digits with only the same two cells left in a unit clear every other candidate from those cells.
  4. Naked TripleThree cells that pool to exactly three digits clear those digits from the rest of their unit.
  5. Hidden TripleThree digits with only three cells to go to in a unit clear every other candidate from those cells.
  6. Naked QuadFour cells that pool to exactly four digits clear those digits from the rest of their unit.
  7. Hidden QuadFour digits with only four cells to go to in a unit clear every other candidate from those cells.

Fish & Wings

  1. SkyscraperTwo two-place columns that share a base row: one roof holds the digit, so cells seeing both roofs lose it.
  2. 2-String KiteA two-place row and a two-place column meeting in one box: the cell where their far ends cross loses the digit.
  3. Turbot FishTwo strong links on one digit, joined end to end: cells that see both free ends lose the digit.
  4. X-WingTwo rows lock one digit into the corners of a rectangle — every other cell of its two columns loses it.
  5. Empty RectangleA box whose digit sits on one row and one column, plus a strong link from that column: one cell loses the digit.
  6. Finned X-WingOne stray place does not kill an X-Wing — it shrinks what you may clear to the fin's box.
  7. SwordfishThree rows lock one digit into three columns — every other cell of those columns loses it.
  8. Finned SwordfishThree lines that would be a Swordfish but for a fin: the cover cells in the fin's box still lose the digit.
  9. JellyfishFour rows lock one digit into four columns, clearing the rest of them — the rarest fish, and when it is worth the hunt.
  10. XY-WingThree two-candidate cells force a digit into one of two pincers — cells seeing both lose it.
  11. XYZ-WingA three-candidate pivot and two pincers trap Z in three cells — only cells that see all three lose it.
  12. W-WingTwo cells with the same pair, joined by a strong link on one digit — cells that see both lose the other digit.

Chains, Colouring & Uniqueness

  1. Simple ColoringChain one digit's strong links and colour them in turn — one colour is true, so a cell seeing both loses the digit.
  2. 3D MedusaSimple Coloring in 3D: bivalue cells carry the colours from digit to digit, and a colour that contradicts itself is false.
  3. Remote PairsA chain of {a,b} cells, each seeing the next: a cell that sees an odd and an even one loses both digits.
  4. XY-ChainA run of bivalue cells pins one digit to one end of the chain or the other — every cell that sees both ends loses it.
  5. Unique RectangleThree corners share a pair inside two boxes — the fourth loses it, or the puzzle would have two solutions.
  6. BUG+1Every empty cell has two candidates except one: that cell holds the digit its row, column and box list three times.

Killer Sudoku

  1. Cage CombinationsList a cage's combinations, cross them with the grid, and clear every candidate no surviving combination can use.
  2. Forced Cells in CagesCross a cage's combinations with its candidates until one cell has a single digit left — then place it.
  3. Innies & OutiesA row, column or box totals 45: take away the cages inside it, and the cells left over must make the rest.
  4. Rule of 45 across Multiple RegionsTwo whole rows hold 90: add up every cage touching them, and the cells sticking out make up the difference.
  5. Killer PairThe cage sum forces two of its cells in one house onto the same two digits: the rest of the house loses them.
  6. Cage SplitA cage crossing a house still counts toward its 45 when the part inside has a fixed sum.
  7. Innie–Outie DifferenceOne cage sticks out by a cell, another sticks in by a cell: the rule of 45 fixes outie minus innie.