How to play sudoku, the Loomoku way
Sudoku has one rule and no arithmetic. This guide covers the rule, your first two techniques with worked examples, how Loomoku handles mistakes and hints, and every technique the game teaches.
Loomoku: Cozy Sudoku is a calm sudoku game for iPhone and iPad from Silvergrove Labs LLC, and its first chapter can be played in a web browser. Everything on this page applies to both.
What are the rules of sudoku?
A sudoku is a 9 by 9 grid divided into nine 3 by 3 boxes. Some squares start with a digit already in them. These are the givens, and they never change. Your job is to fill every empty square so that each row, each column and each 3 by 3 box contains the digits 1 to 9 exactly once.
That is the whole game. Every Loomoku puzzle has exactly one solution, and you can always reach it by logic. You never need to guess.
From chapter 6, the last level of each chapter is an X-Sudoku. The same rule applies, and the two long diagonals must also contain 1 to 9 exactly once.
How do you start a sudoku?
Start with two ideas, the naked single and the hidden single. Chapter one of Loomoku, First Stitches, can be finished with just these two. The boards below are taken from real chapter one puzzles.
Naked single
Look at the highlighted square in row 1, column 8. Check its row, its column and its box, and count the digits you can see. Every digit except 4 is already there, so this square must be 4.
A naked single is a square with only one possible digit left. Crowded rows and boxes are the best place to look for them.
Hidden single
Now flip the question. Instead of asking what can go in a square, ask where a digit can go. In the highlighted box, try to place a 3. The 3s in the neighbouring rows and columns rule out every empty square but one, so the 3 goes in row 2, column 3.
The square may look as if it has several options, which is why this single is called hidden. Scan each box, row and column for a digit with only one home.
Play the first chapterTwenty puzzles in your browser, no account needed.
How does Loomoku play?
Placing digits
Tap a square, then tap a digit on the pad. Each digit key shows how many of that digit are still missing from the board. Hold a digit key to lock it, then tap squares to place it again and again.
Notes
Turn on Notes to pencil small candidate digits into a square. When you place a digit, matching notes in the same row, column and box are removed for you. This can be switched off in Settings.
Slips and lives
If you tap a digit that is already visible in the same row, column or box, Loomoku refuses the move and nothing is lost. A digit that is wrong in a less obvious way is placed, marked, and costs one life. Puzzles give three lives at grades 1 to 7, four at grades 8 to 10, and five at grades 11 and 12. Practice puzzles have no lives.
Out of lives
Keep Going restores one life and clears the wrong digit. It is free once a day and costs 40 Spools after that. Try Again restarts the puzzle from the beginning at no cost.
Stars
A clean solve earns three stars. Placing any wrong digit takes away one star, and going past the Nudge on any hint takes away one star. You always keep at least one.
Hints
A hint has three steps. The Nudge points at where to look and is free. The next step names the technique and explains it, and uses one Hint unless it is a technique you are meeting for the first time. The last step shows the exact move and can apply it. If a wrong digit is on the board, the hint points at that first.
Gentle mode
An easier way to play. Journey puzzles are capped at grade 2 and Daily and Weekly puzzles at grade 3, all as standard sudoku. A wrong digit is marked but costs no life, and hints are free. Players who say they are new to sudoku start in Gentle mode, and it can be switched on or off in Settings.
Tiles, Spools and the Quilt
Each solved board becomes a Tile coloured by the order you filled it in. Wins earn Spools, which buy Hints, streak freezes, board themes, quilt palettes and Tile frames in the Shop. Daily puzzle Tiles fill a Quilt for the month.
How are Loomoku puzzles graded?
Loomoku grades every puzzle from 1 to 12 with a solver that works the way a person does: it always uses the simplest technique available and records each step. The grade tells you the hardest kind of step the puzzle needs, and how often you need it.
Singles only. Fewer givens means a higher grade: at least 40 at grade 1, 34 at grade 2, 30 at grade 3 and 26 at grade 4.
Needs pointing and claiming or a pair. The grade rises with how many such steps the solve takes.
Needs a triple, an X-Wing or an XY-Wing, and rises the same way.
Needs a Swordfish, an XYZ-Wing or simple coloring. Grade 12 needs more of them.
Which grades does each chapter use?
The Journey has 1,000 levels in 50 chapters of 20, and climbs in a gentle sawtooth. Level 10 of each chapter is a Bonus level that pays double Spools the first time you solve it, and level 20 is the Boss at the chapter's top grade.
| Chapters | Grades |
|---|---|
| 1 | 1 and 2 |
| 2 | 1 to 3 |
| 3 | 2 and 3 |
| 4 | 2 to 4 |
| 5 | 3 and 4 |
| 6 to 10 | 3 to 5 |
| 11 to 15 | 4 to 7 |
| 16 to 20 | 5 to 8 |
| 21 to 30 | 6 to 10 |
| 31 to 40 | 7 to 11 |
| 41 to 50 | 8 to 12 |
After level 1,000 the endless Grandmaster Path cycles through grades 9 to 12, with an X-Sudoku every tenth level.
Which techniques does Loomoku teach?
Loomoku's hints can name and explain twelve techniques, the same descriptions you will find in the app. A unit means any row, column or 3 by 3 box.
Gentle Grades 1 to 4
- Naked Single
- A cell has only one digit left that can go in it, because every other digit already appears in its row, column, or box.
- Hidden Single
- Within one row, column, or box there is only one cell where a particular digit can still go, so it must go there.
Steady Grades 5 to 7
- Pointing and Claiming
- All the places a digit can go inside a box line up in one row or column. That digit cannot appear elsewhere in that row or column, or elsewhere in the box.
- Naked Pair
- Two cells in the same unit hold exactly the same two candidates. Those two digits belong to those cells, so they can be removed from every other cell in the unit.
- Hidden Pair
- Two digits can only go in the same two cells of a unit. Those cells must hold those digits, so any other candidates in them can be removed.
Tricky Grades 8 to 10
- Naked Triple
- Three cells in a unit share only three candidates between them. Those three digits are locked into those cells and can be removed from the rest of the unit.
- Hidden Triple
- Three digits are confined to the same three cells of a unit. Any other candidates in those three cells can be removed.
- X-Wing
- A digit appears in exactly two columns in each of two rows, and the columns match. The digit must sit on one of the two diagonals of that rectangle, so it can be removed from the rest of those columns.
- XY-Wing
- A pivot cell with two candidates sees two other two-candidate cells that each share one digit with it. Whichever way the pivot goes, one wing holds the shared third digit, so that digit can be removed from any cell that sees both wings.
Fiendish Grades 11 and 12
- Swordfish
- Like an X-Wing with three rows: a digit's positions in three rows are confined to the same three columns, so it can be removed from those columns elsewhere.
- XYZ-Wing
- A pivot with three candidates sees two wings that each share two of them. The common digit must appear in one of the three cells, so it can be removed from any cell that sees all three.
- Simple Coloring
- Follow a chain of cells where a digit has only two options per unit, colouring them alternately. If one colour appears twice in a unit, that colour is wrong; a cell that sees both colours cannot hold the digit.
Keyboard shortcuts in the browser
On a computer you can play the browser chapter from the keyboard.
- 1 to 9
- Place a digit, or add a note in Notes mode
- Arrow keys
- Move the selection
- Backspace or 0
- Erase
- N
- Notes on or off
- U or Ctrl Z
- Undo
- H
- Hint
- Esc
- Pause