Sudoku Solving Basics: Missing Numbers, Candidates and Local Play
Play native Sudoku on this site. It uses a local board rather than a GameDistribution embed. Start with Easy if you want to practice the deductions below; Medium, Hard and Expert are also available.
The three constraints
A Sudoku board has 9x9 cells, divided into nine 3x3 boxes. Fill each empty cell with a digit from 1 to 9 so that every row, column and box contains each digit exactly once. The given digits cannot be edited.
An empty cell’s candidates are the digits not already present in its row, column or box. A candidate is a possibility, not an answer. Recheck it after other cells change.
Start with a missing number
Suppose a row reads:
1 2 3 | 4 5 6 | 7 8 _The row is missing only 9. Before writing 9, check that the empty cell’s column and box do not already contain it. If they do, an earlier entry is wrong; do not force the row to completion.
Eliminate a candidate using a column
Now consider this partial row:
1 _ 3 | 4 5 6 | _ 8 9The missing digits are 2 and 7. Row information alone leaves both possibilities for each blank. For this example, suppose the other cells in the top-left box are empty, and column 2 contains a given 7 in row 8 with its other cells empty. No other givens constrain the blanks.
The first blank, in column 2, cannot be 7 because that column already contains 7. It must be 2. After entering 2, the only missing number in the row is 7, so the blank in column 7 must be 7. The column and box must still permit it.
This is a deduction from a small example, not a technique that solves every puzzle. When several candidates remain, move to another row, column or box rather than guessing immediately.
Use pencil notes as a scratchpad
Select an empty cell, turn on the pencil tool, then choose a number to toggle its note. Selecting the same number again removes that note. Notes can only be added to empty, editable cells.
Notes are manual: the board does not automatically calculate candidates or remove a candidate from neighboring cells when you place a number. Writing a normal entry clears that cell’s notes. Turn the pencil tool off before entering an answer. Erase clears the selected editable cell’s entry and notes.
What Undo and Check actually do
Undo restores the previous entry or note change in the current puzzle. It does not undo selecting a cell or switching pencil mode, and it cannot recover a puzzle discarded by New game or Restart.
Check compares filled entries with the generated solution and marks incorrect entries. It does not validate pencil notes, explain a deduction or fill an answer. “No incorrect entries found” can appear on an unfinished or empty board; it does not mean the puzzle is complete. Completion requires every cell to be filled correctly.
Use Check after a deduction if you want feedback, but try to explain why the digit fits before relying on the answer comparison.
Before changing puzzles
New game generates another puzzle. Changing difficulty also starts another puzzle. Restart returns the current puzzle to its givens. If you have entries or pencil notes, these actions ask for confirmation; Cancel keeps your current puzzle.
Confirming clears entries, notes and undo history. Progress is held in the current page session, not saved as a resumable game. A refresh or leaving the page loses the current board; there is no game archive to reopen it from.
For a first session, try one row with a single missing digit, then one cell whose row, column and box leave only one candidate. Keep notes for unresolved cells and revisit them after each confirmed entry. Open Sudoku when you are ready to practice.