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What clears the fog?
Level 5 · Outside clues and specials
How to solve fog of war sudoku
Most of the grid starts hidden. Correct digits clear the fog around them.
Fog of war sudoku hides most of the grid at the start. When you place a correct digit, the fog clears from the cells around it and shows any clues underneath.
The hidden clues are still part of the puzzle. You solve with what you can see, and each correct digit reveals more of what you need for the next step.
How setters write it
The same rule, worded the ways you’ll see it on puzzles.
- The grid is partially covered in fog. Placing correct digits will clear the fog from surrounding cells.
- Dynamic fog.
Worth memorizing
- Only correct digits clear fog. A wrong digit leaves it in place, which is itself a clue.
- Clues under the fog still count. You just can’t see them yet.
Solving tips
Start where you can see
Begin with the visible cells and the clues near them. A fair fog puzzle has a logical first digit inside the area that starts clear.
Fog that stays is a clue
Only correct digits clear fog. If you place a digit and the fog around it stays, that digit is wrong, and you can rule it out of that cell.
Read new clues right away
Clearing one cell can show a line, dot or cage edge beside it. A newly revealed white dot next to your 4 means that neighbor is 3 or 5.
Rules reach into the fog
Sudoku rules apply to hidden cells too. A 7 you place in a row rules out 7 in the fogged cells of that row, even before you see them.
A worked example: clearing fog from the first box
A fog puzzle starts with only the top-left box clear. We name cells by row and column, so row 2, column 3 is r2c3. The box has seven given digits and is missing 4 and 9. r2c3 and r3c3 are empty. A white dot joins r3c3 to r3c2, which holds 5. In this puzzle, a white dot means the two digits are one apart.
- The box needs 4 and 9, and only r2c3 and r3c3 are empty. So those two cells hold 4 and 9 in some order.
- The white dot says r3c3 is one away from 5, so it is 4 or 6. The box doesn't need 6, so r3c3 = 4. That makes r2c3 = 9.
- Place the 4 in r3c3. It is correct, so the fog clears from r3c3 and every cell touching it: rows 2 to 4, columns 2 to 4. Five new cells appear: r2c4, r3c4, r4c2, r4c3 and r4c4.
- Place the 9 in r2c3. The cells touching it are rows 1 to 3, columns 2 to 4, which adds r1c4. Six new cells are now clear along column 4 and row 4.
- Read the new cells before your next move. Any given digit, line, dot or cage edge that was hidden there now counts, and it may give the next step in column 4 or row 4.
Practice
Reference table
| Where you place a correct digit | Cells cleared, counting its own |
|---|---|
| A corner of the grid | 4 |
| An edge of the grid, away from the corners | 6 |
| Anywhere else | 9 |
Going deeper
How fog works in this app
In this app, a correct digit clears the fog from its own cell and every cell touching it, including at the corners. Some puzzles set their own reveal areas, so a correct digit in a certain cell clears a set group of cells. A wrong digit clears nothing. Pencil marks don't clear fog either. Only a placed digit counts.
If you erase a correct digit, the fog it was holding back returns, unless another correct digit keeps those cells clear. Given digits that the fog revealed stay on the grid. They fill in by themselves when the fog over them clears. Logic hints are turned off in fog puzzles, because the solver would see through the fog.
Choosing where to clear next
A correct digit on the edge of the clear area reveals more new cells than one in the middle of it. When you have two safe placements, place the one closer to the fog first. It may show a clue you need.
Hidden cells still follow the normal rules. A digit you place in a row rules it out of the fogged cells in that row. Keep candidates for cells you can't see yet, and fill them in as the fog lifts.
When a given digit appears from under the fog, scan its row, column and box right away. It may finish a single you were close to.
When the app can't check digits
The app needs the puzzle's solution to tell right digits from wrong ones. If a fog puzzle loads without one, any digit clears the fog around it, and the app shows a message saying so.
In that case, the fog no longer warns you about mistakes. Check each placement against the rules yourself before you rely on the cells it reveals.
Common mistakes
Treating fogged cells as empty
Fog can hide given digits, lines, dots and cage edges. A cell that looks empty may hold a given digit you can't see yet. Fog can also hide the end of a line or the total of a cage. Don't assume nothing is there.
Solving by testing digits
Since only correct digits clear fog, you can test a digit and see what happens. That skips the logic the puzzle was built around. Use it as a check after you have a reason for the digit.
Expecting pencil marks to clear fog
Corner and center marks don't clear anything. Place a full digit in the cell when you are sure of it.
In mixed-rule puzzles
Fog changes how you read other clues. A line that runs into the fog may continue under it, so don't assume it ends at the edge of the clear area. A killer cage may have its total hidden under the fog, and a dot you can see may join a cell you can't. Rules that cover the whole grid, such as anti-king, anti-knight or diagonals, still apply in fogged cells. Use them to limit hidden cells before you see them. Odd and even marks, dots and cage totals that appear from under the fog count as soon as you can see them. Each new clue that appears is part of the puzzle from the start, so recheck any earlier deduction it touches.
What the video shows
- Most of the grid starts hidden in fog.
- Place a correct digit...
- ...and the fog clears around it, revealing any clues underneath.
Questions solvers ask
What is fog of war sudoku?
A sudoku where most of the grid is hidden at the start. Placing correct digits clears the fog around them.
Do wrong digits clear the fog in sudoku?
No. Only correct digits clear it, so fog that stays after a placement tells you the digit is wrong.
Are there clues hidden under the fog?
Often, yes. Lines, dots, cages and given digits can sit under the fog and still count toward the rules.
Can you solve fog of war sudoku without guessing?
Yes. A fair fog puzzle is built so the visible area always holds a logical next step.
Why did a wrong digit clear the fog?
In this app, that happens when the puzzle loaded without a solution. The app can't tell right from wrong, so any digit clears the fog around it. A message tells you when this is the case.
Do pencil marks clear the fog in fog of war sudoku?
No. Only a placed digit clears fog. Corner and center marks leave the fog in place.
What happens if I erase a digit in fog of war?
In this app, the fog returns over cells that no other correct digit keeps clear. Given digits the fog revealed stay visible.
Foggy Regional Constraints
Regional Constraint Rally
The Positive Diagonal
Bullseye!
Foggy Nonsense
Sum Foggy Quads