Fill the 9×9 grid so every row, column and 3×3 box holds the digits 1 through 9 exactly once.
You're given a partly filled 9×9 grid. Place digits so that each row, each column and each of the nine 3×3 boxes contains every number from 1 to 9 with no repeats. Logic alone solves every puzzle, no guessing required.
Sudoku has a false ancestor and a real one. The false one is the magic square, which it resembles but does not descend from; the real line runs through French newspaper puzzles of the late 19th century, where setters began removing numbers from magic squares to see what remained solvable.
The Paris daily Le Siècle printed a partially completed 9×9 square with 3×3 subsquares on 19 November 1892. It was not sudoku — it used double-digit numbers and needed arithmetic rather than logic — but the skeleton was there. Its rival La France went further on 6 July 1895, refining the puzzle into something very nearly modern and naming it carré magique diabolique, the diabolical magic square.
What the modern form removed was the arithmetic. Once the numbers stopped having to sum to anything, they stopped being quantities at all — nine digits used as nine distinct symbols, which is why the puzzle survives translation into colours, shapes or letters without changing in the slightest.
Sudoku is the classic number-placement logic puzzle. This is an original, free browser version with four difficulty levels, pencil notes, hints and daily and practice puzzles, each guaranteed to have a single solution.