Saturday, March 21

Daily Sudoku

Medium schedule 0:00

Arrow keys to navigate · 1–9 to fill · Del to erase · N for notes

More Sudoku Variants

Master new dimensions of logic with these sudoku twists

Killer Sudoku

Cages must sum to targets

HARD arrow_forward

Sudoku X

Diagonals must also hold 1–9

MEDIUM arrow_forward

Samurai

Five overlapping 9×9 grids

EXPERT arrow_forward

Mini Sudoku

6×6 grid, perfect for beginners

EASY arrow_forward

Sudoku Zen

No timer, no pressure, pure logic

ZEN arrow_forward

Sudoku Strategy Guide

From basic elimination to advanced logical techniques

Naked Singles — The Foundation

A cell with only one possible candidate

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A Naked Single occurs when a cell has only one possible value — every other digit (1–9) is already present in the same row, column, or 3×3 box.

This is the most fundamental solving technique and should always be your first scan. Work methodically through each empty cell and count which numbers are already used in its three units.

How to find them:

Pick an empty cell
List the numbers already in its row
Add numbers from its column
Add numbers from its 3×3 box
If only one number is missing — that's your answer

Example

Cell R5C4 is empty. Its row contains: 1,4,5,7,2. Its column contains: 6,1,0,2,8. Its box contains: 1,3,7,8,4,5.

Combined eliminations: 1,2,3,4,5,6,7,8

∴ R5C4 = 9 ✓

💡 Pro tip: Use the Notes feature to pre-mark all candidates before solving. Naked singles will appear as cells with only one note remaining.

Hidden Singles — One Place Only

When a digit can only go in one cell within a unit

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A Hidden Single occurs when a specific digit can only be placed in one cell within a row, column, or box — even though that cell may have multiple candidates.

The digit is "hidden" because the cell appears to have choices, but only one position in the unit can legally hold that value.

Scanning method:

Choose a digit (e.g., 7)
For each row: find all empty cells where 7 is still possible
If only one such cell exists in that row → place 7 there
Repeat for columns and boxes
Work through all 9 digits systematically

Example

In Row 3, the digit 4 is missing. Check each empty cell's column and box — only R3C7 doesn't have a 4 in its column or box.

∴ R3C7 = 4 ✓

💡 Pro tip: Hidden singles in boxes are often easier to spot visually — scan each 3×3 box for digits that can only fit in one position. This is called box scanning.

Naked Pairs & Triples — Locking Candidates

Two cells sharing the same two candidates eliminate those from the rest of the unit

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A Naked Pair is two cells in the same unit (row/column/box) that together contain exactly the same two candidates — and nothing else. Those two digits must go in those two cells (in some order), so they can be eliminated from every other cell in that unit.

A Naked Triple extends this to three cells sharing a pool of exactly three candidates (e.g., {2,5}, {5,9}, {2,9} — the pool is {2,5,9}).

How to apply:

Find two cells in a unit with identical candidate sets of size 2
Remove those two candidates from all other cells in the unit
This often triggers naked or hidden singles elsewhere

Example

In Column 5, cells R2C5 and R8C5 both have only candidates {3, 7}.

Therefore, 3 and 7 are locked into those two cells. Remove 3 and 7 from all other empty cells in Column 5.

Result: other cells in Col 5 can no longer contain 3 or 7 ✓

💡 Pro tip: Naked pairs are common in expert puzzles. Always scan for them after you've exhausted singles — they're often the key to unlocking a stuck position.

Pointing Pairs — Box/Line Reduction

When a candidate in a box aligns along one row or column

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A Pointing Pair (or Triple) occurs when, within a 3×3 box, all remaining positions for a candidate digit lie in the same row or column. Since the digit must go in that row/column somewhere in that box, it can be eliminated from the rest of that row/column outside the box.

This is the first technique that bridges box logic with line logic — a critical leap in expert-level solving.

Steps:

In each box, locate all positions for a given digit
If they all share the same row → eliminate from rest of that row
If they all share the same column → eliminate from rest of that column

Example

In the top-left box, digit 6 can only go in R1C2 or R1C3 — both in Row 1.

Therefore, 6 cannot appear in R1C4 through R1C9 (the rest of Row 1 outside that box).

Eliminate 6 from all other empty cells in Row 1 ✓

💡 Pro tip: The reverse also works — Box/Line Reduction: if a candidate in a row/column is confined to one box, eliminate it from the rest of that box.

X-Wing — Cross-Row Elimination

A powerful pattern across two rows and two columns

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An X-Wing occurs when a candidate digit appears in exactly two cells in each of two different rows, and those cells fall in the same two columns. The digit forms a rectangle across the grid.

Because the digit must occupy one of the two cells in each row, it locks into those columns — meaning you can eliminate it from all other cells in those two columns.

Identification:

Find two rows where digit X appears in exactly 2 candidates each
Check if those candidates are in the same two columns
If yes → eliminate digit X from all other cells in those columns
The pattern also works with columns eliminating from rows

Example

Digit 4 appears as a candidate in exactly R2C3 & R2C7, and also in R7C3 & R7C7.

These four cells form an X-Wing rectangle. Digit 4 must be in C3 or C7 in both rows.

Eliminate 4 from all other cells in Column 3 and Column 7 ✓

💡 Pro tip: X-Wings are rare but powerful. When you're stuck after trying all simpler techniques, systematically check each digit for the X-Wing pattern.

Swordfish — Three-Tier X-Wing

The X-Wing pattern extended across three rows and three columns

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A Swordfish is the natural extension of the X-Wing to three rows and three columns. A candidate digit appears in 2–3 positions in each of three rows, and those positions are all confined to the same three columns.

The logic is the same: since the digit must be placed once per row across those three columns, it cannot appear anywhere else in those three columns.

When to use it:

Only after exhausting all simpler techniques
When three rows each show the candidate in 2–3 of the same columns
Often unlocks a cascade of simpler moves
Appears frequently in "evil" or "expert" rated puzzles

Advanced Example

Digit 9 in Row 1 is possible at C2, C6. In Row 5 at C2, C6. In Row 8 at C2, C6.

All three rows confine digit 9 to Columns 2 and 6 — this is a Swordfish across 3 rows / 2 columns.

Eliminate 9 from all other cells in Columns 2 and 6 ✓

💡 Mastery tip: There are even higher-order fish patterns — Jellyfish (4 rows/cols) and Squirmbag (5). In practice, if you need these, the puzzle is extreme — consider whether you're missing a simpler technique first.

The Mindful Approach

Solving is a ritual, not a race

The best solvers don't guess — they eliminate. Every correct move can be proven logically. If you find yourself guessing, step back and look for a pattern you may have missed. Use Notes mode liberally, scan systematically, and remember: the puzzle always has a unique solution waiting to be discovered.

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