Rob
Three constraints, one grid
A standard Sudoku has nine rows, nine columns and nine 3 × 3 boxes. Put one number from 1 to 9 in each empty cell so that every row, column and box contains all nine numbers exactly once. The printed givens cannot change. Addition and multiplication are not part of the rules.
Start by checking one empty cell
Imagine a row containing 1, 2, 3, 5, 6, 7 and 8. Its two empty cells must contain 4 and 9. If the column through the first empty cell already contains 9, that cell must be 4. The other cell in the row is then 9. Always check the box too before writing a number.
Candidates are possibilities
For an empty cell, cross out numbers already present in its row, column and box. The numbers left are candidates. A single remaining candidate can be placed. Alternatively, a number may have only one possible position in a row, column or box. Revisit nearby cells after each placement because their candidate sets may change.
A unique answer is not a difficulty rating
A solver can count how many completed grids satisfy the givens. Printable Puzzle Kit publishes puzzles only when that count is one. This proves the answer is determined, but it does not prove which human techniques are needed. We show the clue count separately and use our documented technique rubric for Easy, Moderate and Challenging.
When you get stuck
Check your recent entries against all three constraints and erase uncertain guesses. Start again from the printed givens if needed. The solution page is there when you want it; comparing a single cell can be enough to locate a mistake. A fresh seed gives another puzzle without creating a daily schedule or a separate page.
