Sudoku looks simple: fill a 9×9 grid so every row, column, and 3×3 box contains the digits 1 through 9 exactly once. But that simplicity hides real depth. This guide walks through the techniques solvers actually use, in the order you'll need them, plus a bit of the game's history and why so many people make it part of their daily routine. If you're just starting out, the Beginner Tips page covers habits and common mistakes that matter just as much as the techniques below.
Almost every puzzle — even hard ones — starts by clearing out the easy cells first. These three techniques will get you through the opening of any grid.
Pick a number, say 7, and scan each 3×3 box in turn. If two of the three rows (or columns) crossing that box already contain a 7 somewhere else in the grid, and the box has only one empty cell left in the remaining row/column, that cell must be the 7. This is the fastest way to fill in a puzzle's first clues.
Look at a single empty cell and list every number it could legally hold, based on what's already in its row, column, and box. If only one number survives that process, it belongs there — no further reasoning needed.
Sometimes a cell still looks like it has several options on its own, but one of those candidate numbers can't fit anywhere else in that row, column, or box. When a candidate is "hidden" like that — only possible in one spot even though the cell itself allows others — it's forced there.
Once scanning and singles stop making progress, these pattern-based techniques unlock the next layer of the puzzle.
If two cells in the same row, column, or box both have their candidates narrowed down to the exact same two numbers, those two numbers must occupy those two cells — in some order. That means every other cell in that row, column, or box can safely have those two numbers eliminated from its candidate list. The same logic extends to three cells sharing three candidates (a naked triple).
The mirror image of a naked pair: if two candidate numbers only appear in the same two cells within a unit — even if those cells have other candidates cluttering them up — those two numbers must go in those two cells. Every other candidate can be stripped out of those two cells.
If a candidate number within a box only appears in cells that all sit in the same row (or column), that number can be eliminated from the rest of that row outside the box — it's "pointing" out of the box. The reverse also works: if a candidate in a row is confined to a single box, it can be eliminated from the rest of that box.
Expert-level puzzles occasionally need these — they take longer to spot, but they're what separates a stuck grid from a solved one.
If a candidate number appears in exactly two cells in each of two different rows, and those cells line up in the same two columns, they form a rectangle. That number can then be eliminated from every other cell in those two columns — the four corners "lock" the candidate in place.
The same idea as an X-Wing, extended to three rows and three columns instead of two. It's rarer and harder to spot, but the logic is identical.
A chain of three cells, each with exactly two candidates, linked so that one shared number rules out a candidate in a cell that "sees" all three. It's the technique most solvers reach for once pairs, triples, and X-Wings run dry.
In SUDOKU — Premium Edition, difficulty isn't just about how many clues are given — it's about which techniques a puzzle actually requires to finish:
If you're moving up a difficulty and getting stuck, it's almost always a sign to look for the next technique on this list rather than to keep re-scanning what you've already checked.
Sudoku's roots trace back to the 18th century, when Swiss mathematician Leonhard Euler experimented with "Latin squares" — grids where each symbol appears once per row and column. The modern 9×9 puzzle with its added 3×3 box constraint was created much later, by retired American architect Howard Garns, and first published in 1979 by Dell Magazines under the name "Number Place."
The puzzle found its true home in Japan, where puzzle publisher Nikoli introduced it in 1984 and gave it the name we use today — Sudoku, short for sūji wa dokushin ni kagiru ("the digits must remain single"). It stayed largely a Japanese pastime until the mid-2000s, when retired judge Wayne Gould wrote a program to generate puzzles quickly and pitched them to The Times of London in 2004. The puzzle exploded internationally within a year, and newspapers, books, and apps worldwide have carried it ever since.
Sudoku has stuck around for a reason. A few things players consistently mention: