Sudoku Tutorial Step 3: How Start Solving Sudoku: Scanning and Observation

Welcome back to Sudoku Fun! In Article 1 we learned what sudoku is, and in Article 2 we explored the grid structure. Now it's time to actually start solving. In this guide, you'll learn the most fundamental skill in sudoku: scanning and observation — the art of seeing what's right in front of you.

Practice Scanning — Try This Easy Puzzle!

How to play:
Fill empty cells with numbers 1–9. Each row, column, and 3×3 box must contain all digits 1–9.
Red = duplicate. Green = your answer. Gray = given clue.

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# How to Start Solving Sudoku: Scanning and Observation

Quick Summary: Sudoku is not "calculated" — it is "seen." The most important skill for any sudoku player, from beginner to expert, is the ability to scan the grid and observe what is already there. This guide teaches you the two fundamental scanning techniques — the Full Scan and Cross-Hatching — plus how to avoid the most common beginner mistake: "looking but not seeing."

🧠 The Logic Behind It — Observation as the First Logical Step: Before you can deduce anything, you must gather data. In formal logic, this is called premise collection — listing all known facts before drawing conclusions. In sudoku, the "known facts" are the given numbers on the grid. A systematic scan ensures you collect every available premise before you begin reasoning. Skipping this step is like a detective trying to solve a case without first examining the crime scene.

# Why Scanning Matters: "Looking" vs "Seeing"

When beginners first open a sudoku puzzle, their eyes typically dart around randomly — they look at a cell, guess a number, check for conflicts, and repeat. This is trial and error, not logical solving.

Experienced solvers do something completely different. Before they fill in a single cell, they scan the entire grid systematically. They are not just "looking" — they are actively observing with a clear method.

▲ Left: random eye movement misses cells. Right: systematic scanning covers every cell once and only once.

# Technique 1: The Full Scan — "1 to 9" Method

This is the simplest and most reliable scanning method. It works like this:

Step 1: Pick a number. Start with 1.

Step 2: Look at the entire grid and find every cell that already contains that number.

Step 3: For each row, column, and 3×3 box that already has that number, mentally note: "This row/column/box doesn't need this number anymore." This is the key insight — you are searching for where the number cannot go, which reveals where it must go.

Step 4: If the number is missing from a row, column, or box, check whether there is only one possible cell left for it. If so — write it in!

Step 5: Move to the next number: 2, 3, 4... all the way to 9.

Step 6: Repeat! After finishing 1 through 9, start again from 1 — because numbers you placed in later steps may open up new opportunities for earlier numbers.

▲ The Full Scan method: cycle through numbers 1–9, several times, each pass building on the last

# Technique 2: Cross-Hatching — Zone Intersection

Cross-hatching is a more advanced scanning technique that focuses on a single 3×3 box at a time. Instead of scanning the whole grid, you focus on one box and ask:

"For this specific box, which cells can possibly contain a given number?"

Here is the logic: a number's position in a box is constrained by the same number appearing in neighboring rows and columns. When the number 5 appears in a row outside the box, every cell in that same row inside the box cannot contain 5. By drawing "lines" from neighboring instances of a number and finding their intersection in the target box, you often find exactly one cell left — and that's where the number goes.

▲ Cross-hatching: the intersection of a row-blocked zone and a column-blocked zone leaves exactly one cell for the number in the target box.

💡 Cross-hatching is more efficient than full scan for Medium+ puzzles: Once you have scanned the full grid 1-2 times and placed the obvious numbers, cross-hatching takes over. You focus on one box, find candidates using row/column exclusion lines, and place numbers that the full scan would have missed. Experienced solvers use full scan to "clear the easy ones" and cross-hatching to "crack the harder ones."

# The Most Common Beginner Mistakes (and How to Avoid Them)

Even with the right techniques, beginners tend to fall into a few predictable traps. Recognizing these will save you a lot of frustration:

🔴 Mistake 1: Skipping the Scan — Jumping Straight to Filling
This is like starting a jigsaw puzzle by randomly grabbing pieces without first looking at the picture on the box. Solution: Force yourself to do at least one full 1-9 scan before placing your first digit. Make it a habit.

🔴 Mistake 2: Tunnel Vision — Staring at One Cell for Too Long
When you fixate on a single cell ("What can possibly go here?"), you lose sight of the bigger picture. Solution: If a cell doesn't reveal its number within 5 seconds, move on. Return to it after the next scan.

🔴 Mistake 3: Forgetting to Rescan After Each Fill
Every number you place changes the board. A new 5 in column 3 might unlock a single candidate in box 7. Solution: After placing any digit, quickly rescan the row, column, and box you just affected. Don't restart the full 1-9 scan — just check the local impact.

⚠️ The "I'll Just Guess" Trap: When you cannot immediately see where a number goes, your brain wants to guess. Resist this! Guessing breaks the logical chain. If you guess wrong and don't realize it, you may spend 15 minutes building on a false assumption only to hit a contradiction that forces you to erase everything. Every single cell in a proper sudoku can be logically deduced — if you cannot see the logic yet, it means you haven't scanned thoroughly enough, or you need a more advanced technique (which we will cover in later tutorials).

# Putting It All Together: A Step-by-Step Scanning Routine

Here is a concrete routine you can follow for every new sudoku puzzle. Try it on the interactive puzzle at the top of this page:

✅ Your Scanning Routine Checklist:

Step 1 — Full Scan, Pass 1: Go through numbers 1→9. For each number, look for rows/columns/boxes where the number already exists, and check if it creates any "must go here" situations. Place any obvious numbers you find.

Step 2 — Full Scan, Pass 2: Repeat 1→9. New numbers from Pass 1 will have created new opportunities.

Step 3 — Cross-Hatch Each Box: Focus on one 3×3 box at a time. Use row and column exclusion lines from numbers outside the box. This will catch what the full scan missed.

Step 4 — Full Scan, Pass 3: After cross-hatching all 9 boxes, do one more 1→9 pass. At this point you should have placed most of the easy and medium-difficulty numbers.

Step 5 — Pencil Marks (If Stuck): If you are stuck after 3 full scans and cross-hatching, start noting down candidate numbers in the remaining empty cells. This is the topic of a future tutorial, but you can use it now: for each empty cell, write down which numbers 1-9 are still possible based on its row, column, and box.

▲ Your scanning routine: 3 full-scan passes + cross-hatching = most Easy/Medium puzzles solved!

# Frequently Asked Questions

Q: How long should a full scan take?
A: For beginners, a complete 1→9 scan of all 9 digits takes about 2–3 minutes. With practice, you will get this down to 30–60 seconds. Remember: scanning time is not wasted — it saves you the much larger time cost of guessing and backtracking.

Q: When should I use full scan vs cross-hatching?
A: Use full scan first (Passes 1 and 2) to place the obvious numbers. Then switch to cross-hatching when the easy placements dry up and you are working on a specific box. Return to full scan (Pass 3) after cross-hatching to catch any new easy placements.

Q: What if I scan everything and still cannot find any number to place?
A: This is normal for harder puzzles! It means the puzzle requires techniques beyond basic scanning — such as pencil marks, naked singles, or pairs. These are exactly the techniques we will cover in the next several tutorials. For now, click "New Game" on the puzzle above to try a fresh Easy puzzle, which should be solvable with scanning alone.

Q: Do expert solvers use the same scanning techniques?
A: Yes — but they have practiced so much that the process becomes automatic and nearly instant. What takes you a conscious 1-9 count, an expert does in a split second through trained pattern recognition. Every expert started exactly where you are now: deliberately scanning digit by digit.

# Key Takeaways

📝 Summary — 3 Things to Remember:
🔹 Scan before you solve. Never jump straight to filling cells. A systematic scan reveals the easy placements and gives you the "lay of the land."
🔹 Full scan (1→9) and cross-hatching (box-by-box) are complementary. Full scan for the big picture; cross-hatching for focused deduction. Use both.
🔹 If you cannot see a logical placement, keep scanning. Guessing is not solving. Every cell in every proper sudoku can be deduced — you just haven't seen the constraint yet. Be patient. Observation is a skill that improves with practice.

# Practice Challenge

Use the interactive puzzle at the top of this page. Before typing anything, do two complete 1→9 full scans. Only after the second scan, start filling numbers. Can you solve the entire puzzle using only the scanning techniques from this guide? Give it a try!

# Next Step

Now that you know how to systematically scan a sudoku grid, the next skill is learning to fill in cells where only one number is logically possible. This is called the Single Candidate Rule — and it is your most powerful tool as a beginner.

👉 Next: The Single Candidate Rule: Filling the Obvious Numbers

Did this scanning guide help you solve faster? Tell us in the comments — what was the first number you placed after scanning? 🧩

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