Sudoku

Sudoku is a logic-based number-placement puzzle with a deceptively simple goal: fill a 9×9 grid so that every row, column, and 3×3 box contains all digits from 1 to 9 exactly once. While modern competitions emphasize speed, the enduring appeal of Sudoku lies in structured deduction. Every proper puzzle is an exercise in pure reasoning, where each placed number narrows down the remaining possibilities according to rigorous geometric constraints.

How difficulty is graded by technique

Many online generators grade puzzles solely by counting clues. This is misleading: a board with 24 clues can be straightforward to solve, while another with 30 clues can demand sophisticated pattern recognition. Our generator verifies every board against a human-technique solver, classifying puzzles by the most complex technique necessary to reach a solution:

1. Naked Singles and Hidden Singles (Easy): A Naked Single occurs when eight distinct digits already occupy the peers of a cell (its row, column, and 3×3 box), leaving only one eligible number. A Hidden Single occurs when a digit can only fit into a single cell within an entire row, column, or box, even if that cell still has multiple candidates.

2. Pointing Pairs and Box/Line Reduction (Medium): Locked candidate techniques exploit unit interactions. If a digit appears as a candidate only within a single row of a 3×3 box, that digit cannot appear elsewhere in that row outside the box (Pointing Pair). Conversely, if a candidate in a row is confined to a single box, it can be eliminated from the remainder of that box (Box/Line Reduction).

3. Naked and Hidden Pairs and Triples (Hard): When two cells in the same row, column, or box contain the exact same two candidates (such as {3, 7}), those two digits are locked into those two cells. They can be safely erased from all other cells in that unit. Hidden pairs apply the dual logic: when two digits appear in only two cells of a unit, all other candidates can be discarded from those cells.

4. X-Wing and Simple Chains (Expert): An X-Wing pattern forms when a candidate appears in exactly two cells across two parallel rows, and both cells share the exact same two columns. The candidate must occupy opposite diagonal corners of the resulting rectangle, allowing you to eliminate the digit from the rest of those two columns. Simple Chains trace alternating strong links across conjugate pairs to uncover contradictions and color traps.

Strategy and candidate tracking

When progressing beyond Easy puzzles, maintaining candidate notes is essential. Toggling pencil mode with the N key allows you to mark potential digits without committing to an entry. Placing a solved number automatically purges that digit from the pencil marks of all adjacent peers, keeping your board clear and focused on the remaining logical steps.

Controls

Arrow keys / WASD	Move the active cursor around the 9×9 grid.
1 - 9	Enter a number into the selected cell or toggle candidate note.
0 / Backspace / Delete	Erase entered number or candidate notes from cell.
N	Toggle pencil notes mode on or off.
U / Ctrl+Z	Undo the previous move.
Ctrl+Y	Redo the previously undone move.