⚡ Sudoku Quick-Reference Cheat Sheet

Your complete solving companion — keep it open while you play or print it out.

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1. The Core Objective

The one rule that governs everything

Fill the 9x9 grid so that every Row, Column, and 3x3 Block contains the numbers 1 through 9 exactly once. No number may repeat in any house.

2. Beginner's "First Moves" Checklist

Work through these in order at the start of every puzzle

Cross-Hatching (Scanning)

Pick a number (e.g., 1) and "block out" rows/columns where it already exists to find the only empty spot in a 3x3 block. Repeat for all numbers 1–9.

Full House
3 7 1 9 5 2 4 8 6
This row already contains 1, 2, 3, 4, 5, 7, 8, 9. Only 6 is missing — it must go in the green cell. No logic required beyond counting.

Always check for full houses before doing anything else. Also works for columns and boxes.

Full House

Look for any row, column, or block with 8 cells filled. The 9th cell is guaranteed — it must be the missing number. Always grab these first!

Naked Single

If a cell has only one possible candidate left because all other numbers (1–9) are visible in its row, column, or block — fill it immediately.

Hidden Single

If a number can only fit in one specific cell within a row, column, or block (even if that cell has other candidates), it must go there.

Hidden Single
1 4 9 7 2 8 3 6 6 6 5 6 8 6
In the centre box (purple), the number 6 is blocked from every cell except the bottom-right (green) by the 6s already placed in surrounding rows and columns. Even though that cell has other candidates, 6 must go there.

Hidden singles are easy to miss. Always ask: "where can this number go in this box?" — not just "what goes in this cell?"

3. Essential Solving Techniques

From beginner scanning to intermediate logic

TechniqueWhen to UseHow It Works
Scanning At the very start Scan blocks in groups of three (chutes) to see where numbers repeat and force placement.
Pencil Marks When stuck Write small "candidate" numbers in the corners of empty cells. Numbers 1–3 top, 4–6 middle, 7–9 bottom.
Naked Pair Middle of the game Two cells in one house contain the same two candidates (e.g., 2, 7). Remove 2 and 7 from all other cells in that house.
Naked Triple Medium/Hard puzzles Three cells in one house share exactly three candidates between them. Eliminate those three from the rest of the house.
Pointing Pairs Middle of the game If a number only fits in one row within a 3x3 block, it cannot appear elsewhere in that same row outside that block.
Box-Line Reduction Middle of the game The reverse of Pointing Pairs. If a candidate in a row/column falls entirely within one block, remove it from the rest of that block.
X-Wing Hard/Expert puzzles A candidate forms a rectangle across two rows and two columns. Eliminate that candidate from those two columns in all other rows.

4. Sudoku Dos and Don'ts

The habits that separate good solvers from great ones

✅ DO

  • Use a pencil and eraser when playing on paper.
  • Start with the most "filled-in" areas of the grid first.
  • Check the row, column, AND block before placing any number.
  • Use pencil marks liberally — they're the pro's secret weapon.
  • Walk away if you're stuck. Fresh eyes see new patterns.

❌ DON'T

  • Never guess. Every move must be 100% logically proven.
  • Don't focus only on 3x3 blocks — remember long rows and columns.
  • Don't write messy pencil marks. Use a consistent layout.
  • Don't give up — there is always a logical path forward.
  • Don't skip cells that "look" solved — verify them.

5. Essential Terminology

The vocabulary every solver needs to know

House
A collective term for any Row, Column, or 3x3 Block. Each house must contain 1–9 exactly once.
Given
The pre-filled numbers provided at the start of the puzzle. They are fixed and cannot be changed.
Candidate
A possible number for a specific empty cell, based on what is missing from its row, column, and block.
Conflict
When the same number appears twice in the same house. This means a mistake has been made somewhere.
Chute
A set of three aligned blocks — either three horizontal blocks (a row-chute) or three vertical blocks (a column-chute).
Deadly Pattern
A board state that would create two valid solutions. In a correctly constructed puzzle, this should never occur and can be used as a deduction tool.
Bifurcation
The technical term for guessing. It means picking a number and following the logic chain to test it. Avoided by purists.

Want the full glossary? Visit our Complete Sudoku Glossary page for every term from A to X-Wing.

🚀 Your Path to Mastery

LevelKey FocusMost Used Technique
🟢 BeginnerScanning & RowsCross-hatching + Full House
🟡 IntermediatePencil MarksNaked Pairs & Triples
🟠 AdvancedGrid InterconnectivityX-Wings & Pointing Pairs
🔴 ExpertMulti-House LogicSwordfish, XY-Wings & Uniqueness