Sudoku: The Complete Strategy Guide

Sudoku is the world's most-played logic puzzle for a reason: the rules fit on a Post-it, but the techniques scale from coffee-break easy to genuinely challenging. The Puzzle Page app runs a fresh 9x9 Sudoku every day — sometimes a 6x6 variant for a quicker solve — and the same techniques work for both.

This guide takes you through every named Sudoku technique in roughly the order you'll need them: from naked singles you'll find in 5 seconds to X-Wings you'll spot once a week. By the end you'll have a clear mental ladder — when to use which technique, and when to escalate.

How Sudoku Works

Fill a 9x9 grid with the digits 1 through 9 so that every row, every column, and every 3x3 box contains each digit exactly once. Some cells start with given digits; the rest are blank for you to deduce.

That's it. Every Sudoku rule and technique is a consequence of those three constraints. Every digit has exactly one valid position in each row, each column, and each 3x3 box — your job is to find it.

The 6x6 variant

Puzzle Page occasionally publishes 6x6 Sudokus using 2x3 boxes (six 2-tall, 3-wide boxes). All the techniques below work; just substitute "6" for "9" everywhere and "2x3 box" for "3x3 box."

Reading the Grid Like an Expert

Beginner Techniques

Naked singles

If a cell has only one digit that can possibly go there (because the row, column, and box already contain every other digit), that digit is forced. Scan every empty cell — many easy puzzles solve entirely on naked singles.

Hidden singles

If a digit can only go in one cell within a row, column, or box (because every other cell in that region is blocked), that's a hidden single. Even when the cell has many candidates, the digit might have only one home.

The crosshatching technique

Pick a digit, say 4. Look at each 3x3 box that doesn't yet contain a 4. Use the rows and columns that already have a 4 to eliminate cells. Often only one cell in the box can take the 4 — that's the placement. Repeat for every box, then move to the next digit.

Intermediate Techniques

Naked pairs and triples

If two cells in the same row, column, or box can both only contain the same two candidates (say, {3, 7}), those two digits are locked to those two cells. Eliminate 3 and 7 from every other cell in that region.

The same idea extends to triples: three cells, three shared candidates. Naked pairs are common; triples are less common but devastating when you spot them.

Hidden pairs and triples

The mirror of naked pairs. If two digits can only go in two cells within a region, those two cells must contain those two digits — all other candidates in those cells are eliminated. Useful when a cell looks like it has many options but actually doesn't.

Pointing pairs (box-line reduction)

If within a single 3x3 box, all the candidates for a digit lie in the same row, that digit must occupy one of those cells. Eliminate that digit from the rest of that row outside the box. Same idea for columns.

Box-line reduction (the reverse)

If within a row or column, all the candidates for a digit lie in a single 3x3 box, that digit must end up in that box — eliminate it from the other cells of that box.

Advanced Techniques

X-Wing

The first "advanced" technique most solvers learn. Find a digit that has exactly two candidate cells in two different rows, and those cells line up in the same two columns. The digit must occupy two of those four corner cells diagonally — meaning it can be eliminated from every other cell in those two columns.

Swordfish

An X-Wing extended to three rows and three columns. Rarer than X-Wing but follows the same logic: if a digit has its candidates confined to the same three columns across three rows, you can eliminate it from those columns elsewhere.

XY-Wing

Three cells, three pairs of candidates, forming a chain (the "pivot" shares one candidate with each "wing"). If any cell sees both wings, you can eliminate the shared candidate from it. This is the gateway to genuinely hard puzzles.

The contradiction test

When stuck, pick a cell with only two candidates, assume one is correct, and chain-forward. If you hit a contradiction, the other candidate is forced. This is a last-resort technique — if you're using it often, you're missing easier deductions.

Common Mistakes to Avoid

• Not pencilling candidates. Trying to keep all candidates in your head past the easy stage is a recipe for errors. Pencil the candidates in every empty cell once you exhaust naked singles.
• Skipping the digit-by-digit scan. Cell-by-cell scanning misses crosshatching opportunities. Always do digit-by-digit passes too.
• Jumping to guesses too early. If you're guessing on a Puzzle Page Sudoku, there's almost always a technique you missed. Re-pencil candidates and look for naked/hidden pairs.
• Ignoring the box. Beginners check rows and columns but forget the box. The box is the third constraint and often the most useful one.

How Sudoku Compares to Other Number Puzzles

Sudoku is the original Latin-square puzzle. Futoshiki is Sudoku without boxes but with inequality constraints. Kakuro swaps Sudoku's region constraints for arithmetic sums. Cross Sum is a cleaner arithmetic puzzle that's a great warm-up.

If you enjoy Sudoku's pure deduction, you'll also love Os and Xs — it's a binary logic puzzle that rewards the same scanning and propagation habits.

Frequently Asked Questions