Rules & Strategies

Pick any digit — say, 7. Find every row and column that already contains a 7. In each 3×3 box that does not yet have a 7, the rows and columns carrying existing 7s are eliminated. If only one cell remains in the box after elimination, place the 7 there.

Repeat this scan for all nine digits before moving to harder techniques. On easy puzzles, scanning alone often solves 60–70% of the board.

Look at any empty cell. List which digits cannot go there — every digit that already appears in the cell’s row, column, or box is eliminated. If eight of the nine digits are eliminated, only one digit remains: that is the Naked Single. Place it immediately.

This is the most common deduction in easy and medium puzzles. Train yourself to spot cells with only one possibility before working harder techniques.

Look at a row, column, or box. For each digit, find all the cells where it could legally go. If a digit can go in only one cell within that group — even if that cell appears to have multiple possibilities — the digit must go in that cell. This is the Hidden Single.

Example: In a row, the digit 4 is legally possible in three cells. In the box containing those cells, two of the three positions are already eliminated by a 4 in an adjacent box. The remaining cell is the only legal home for 4 in both the row and the box — place it.

For each unfilled cell, mentally (or with the Mark feature) list all digits that are still candidates. Apply all three constraints simultaneously — row, column, and box — to reduce the candidate list. A cell whose list shrinks to one digit is a Naked Single. A digit whose candidate cells within a group shrink to one is a Hidden Single.

This is not a single trick but a disciplined work habit: never guess until elimination has been exhausted. Most published “moderate” puzzles are fully solvable by elimination alone.

If two cells within the same row, column, or box each have exactly the same two candidate digits (and no others), those two digits must go in those two cells — in some order. This means both digits can be eliminated from every other cell in that shared row, column, or box.

Example: Cells A and B in the same box both list only {3, 8} as candidates. You cannot yet determine which cell gets the 3 and which gets the 8 — but you know with certainty that no other cell in the box can hold either digit. Remove 3 and 8 from all other candidate lists in the box.

Within a 3×3 box, find a digit whose only candidate cells all fall in the same row or the same column. Because the digit must go in one of those cells, it cannot appear anywhere else in that row or column outside the box. Eliminate it from the rest of the row or column.

Example: In the top-left box, the digit 6 can only go in row 2 (both possible cells are in row 2). Therefore, 6 cannot be in any other cell of row 2 across the entire board. Remove 6 from the candidate lists of all other row-2 cells.

When applying intermediate or advanced techniques, use the M key in the TNT Sudoku app to mark cells you have placed but are not yet confident about. A marked cell displays in blue italic underline — a visual reminder that this digit was your best current deduction, not a certainty.

In pencil-and-paper Sudoku, players write small candidate digits in each cell’s corner. The Mark feature serves a similar purpose: it flags cells for review. When a conflict appears (red highlight), trace back through your marked cells to find where the chain of deductions went wrong.

When all logical techniques have been exhausted and multiple cells still have two or more candidates, you may need to hypothesize: choose a cell with exactly two candidates, place one, and continue solving. If a contradiction appears (a row, column, or box requires a digit that is no longer available), the hypothesis was wrong — the other candidate is correct.

This is a valid strategy but should always be a last resort. Most Sudoku puzzles rated “expert” or lower are solvable without guessing. If you reach trial and error quickly, more scanning and elimination are usually still available.