Sudoku

Where Sudoku comes from

The grid everyone thinks of as Japanese was born in Indiana. In 1979, a retired architect named Howard Garns published a puzzle called Number Place in Dell Pencil Puzzles & Word Games, quietly and without a byline. It crossed the Pacific in 1984, when the Japanese publisher Nikoli picked it up, trimmed the name to Sudoku (roughly, "the digits must be single"), and added the craftsmanship the genre is now known for: fewer givens, placed with symmetry. The world caught up twenty years later. Wayne Gould, a retired judge from Hong Kong, found a Nikoli book in a Tokyo shop in 1997, spent years writing a program to generate puzzles, and talked The Times of London into printing one in November 2004. Within a year Sudoku was in newspapers on every continent — arguably the fastest global spread any pencil puzzle has ever managed, for a design that had been sitting in plain sight for a quarter century.

How to get better at Sudoku

Scan by digit, not by cell. Pick a number and sweep the board for boxes where its row and column neighbors leave exactly one legal cell — the classic cross-hatch. This finds hidden singles, the workhorse deduction of every easy and medium grid, far faster than interrogating empty cells one at a time.

Learn to see naked singles as you go. When a cell's row, column, and box together rule out eight digits, the ninth writes itself. On our Easy boards, with 62 givens and only 19 blanks, singles alone will usually carry you to the end; on Very Hard, with 35 givens, they are just the opening act.

When singles dry up, hunt pairs. Two cells in one unit that share the same two candidates lock those digits between them, sweeping them from every other cell in that unit. The hidden version — two digits that fit only the same two cells — is just as strong and easier to miss.

Use box-line reduction. If a digit's only homes inside a box lie along one row, that digit is spoken for in that row everywhere outside the box. This is the technique that quietly cracks most Hard grids.

Keep your notes honest. Pencil marks are only as good as their upkeep — a stale candidate is worse than none. On the two hardest levels, note fully and prune after every placement; on Easy, skip notes entirely and train your eye instead.

Never guess. Every board has one solution reachable by reason. The red conflict flag is a seatbelt, not a strategy — if you need it, back up and find the deduction you skipped.

A round in the life

You are six minutes into a Medium board and the singles have gone quiet. So you pick a digit and sweep. Two placed sevens stand in the upper band — one row each — and between them they cut the third row of the top-right box down to a single open cell. Seven goes in, no note required. That placement tightens its column: a cell in the center box that was holding four candidates now holds two, and its neighbor holds the same two. A naked pair. You strike those digits from the rest of the box, and a hidden single surfaces two cells away, the kind you'd have stared past a minute ago. Three entries in thirty seconds, each one forced. The grid isn't harder than you — it's just waiting for you to ask the right question.