Sudoku
puzzleHow to play Sudoku
I fill the 9 by 9 grid so every row, every column, and each of the nine 3 by 3 boxes holds the digits 1 to 9 with no repeats. The bold given numbers are my starting clues and I cannot change them.
I tap a blank cell to select it, then tap a number on the pad below or type 1 to 9 on my keyboard. On desktop I can also move around with the arrow keys and clear a cell with Backspace or the erase key. Cells that clash turn red so I can spot my mistakes fast.
I hit Check any time to see if I am on track, Solve if I want to reveal the answer, and New Game to start a fresh puzzle whenever I like.
About Sudoku
Sudoku's family tree is a world tour: an American architect, Howard Garns, invented 'Number Place' in 1979; Japanese publisher Nikoli refined and renamed it in the 1980s (sudoku roughly means 'single digits'); and a retired New Zealand judge, Wayne Gould, spent six years writing a generator and sold The Times of London on it in 2004, igniting the global craze.
The rules never change: place 1-9 so every row, column and 3x3 box contains each digit exactly once. Despite the numbers, there is no arithmetic, Sudoku is pure constraint logic, and a proper puzzle has exactly one solution reachable without guessing. Mathematicians even proved a valid puzzle needs at least 17 starting clues.
Sudoku technique ladder
- Scan digit by digit: pick a number, find every row, column and box that already contains it, and see which cells are forced.
- Hunt naked singles (cells with one possible digit) and hidden singles (digits with one possible cell in a unit) first, they solve most easy grids alone.
- Pencil-mark candidates in hard puzzles; the notation is not cheating, it is the game.
- Learn naked pairs: two cells in a unit sharing the same two candidates eliminate those digits everywhere else in the unit.
- Never guess. If you are stuck, a legitimate deduction is waiting, guessing corrupts the whole grid when wrong.
FAQ
Do I need to be good at math for Sudoku?
No arithmetic is involved at all, the digits are just nine distinct symbols and could as easily be letters or shapes. Sudoku is an exercise in elimination logic: what CAN'T go here does all the work. That is why it hooks word-puzzle people and math people equally.
What is the difference between easy and hard Sudoku?
Clue placement, not clue count alone. Easy grids fall to singles, scanning each digit; hard grids force chained techniques like pairs, pointing candidates and X-wings, where several deductions must stack before any cell resolves. The rules stay identical; the inference depth grows.
Is guessing ever required?
In a well-formed puzzle, never, every grid here has a unique solution reachable by logic alone. If you find yourself wanting to guess, some elimination is still hiding, usually a hidden single or a pair interaction you have not marked yet.
What are the pencil marks for?
They record each empty cell's remaining candidates so eliminations become visible: when a unit shows two cells sharing the same two candidates, or a digit confined to one row inside a box, the marks reveal it. From medium difficulty up, good notation IS the solving.
