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Sudoku Basic Techniques: Scanning, Cross-Hatching & Pencil Marks

If you've read our beginner's guide, you can already finish Easy puzzles by filling in the obvious cells. But what happens when the obvious moves run out and the grid still has thirty empty squares? You don't guess — you switch to techniques. These three foundational skills (scanning, cross-hatching, and pencil marks) will carry you through every Easy puzzle and deep into Medium territory.

Technique 1: Scanning

Scanning is the habit of looking at the grid systematically instead of randomly. There are three flavors, and strong players cycle through all of them on every puzzle.

Row and column scanning

Pick a digit — say, 5. Now sweep across each row, one by one, asking: "Where can a 5 go in this row?" Most rows already contain a 5, so you skip them instantly. For the rows that don't, check each empty cell: is this column free of 5s? Is this 3×3 box free of 5s? Often only one cell passes both tests, and you've found a placement.

Then do the same for columns: pick a digit, sweep down each column, and test the empty cells. It feels slow at first, but after a few puzzles your eyes start doing it almost automatically.

Box scanning

Now scan box by box. Look at one 3×3 box and ask which digits are missing from it. For each missing digit, check the rows and columns crossing that box — they rule cells out fast. Boxes are where beginners find the most missed opportunities, because it's easy to forget they're a constraint at all.

The 1-through-9 inventory

Here's a power version of scanning: go digit by digit from 1 to 9 across the whole grid. For each digit, find every placement you can. This "inventory sweep" catches cells that random looking misses, and it doubles as a built-in error check — if you ever find a digit with nowhere legal to go, something earlier went wrong.

Practice tip: On SudokuPuzzle.pro, turn on notes mode and do one full 1–9 sweep at the start of every Medium puzzle. You'll be amazed how many cells fall.

Technique 2: Cross-Hatching

Cross-hatching is scanning with a specific target: placing one digit in one box by using the rows and columns that cross it. It's the single most useful beginner technique, so let's walk through it slowly.

Imagine the top-middle box (rows 1–3, columns 4–6). You want to know where the digit 7 goes in that box:

  1. Look at row 1. Suppose there's already a 7 sitting over in column 8. That means row 1 can't hold another 7 — so within our box, the top row is ruled out.
  2. Look at row 2. Suppose a 7 already sits in column 2. Row 2 is ruled out too.
  3. The 7 for this box must be in the only remaining row: row 3.
  4. Now check columns 4, 5, and 6 in row 3. If two of those cells are already filled with other numbers, the 7 goes in the last open one. Done.
Box (rows 1-3, cols 4-6):        Row 1 has a 7 in col 8  →  row 1 ruled out
                                 Row 2 has a 7 in col 2  →  row 2 ruled out
  .  .  .                        ⇒ the 7 must be in row 3 of this box
  .  .  .
  .  X  .

Notice what happened: you never "solved" a cell directly — you eliminated everywhere the digit couldn't go until one home remained. That's the heart of Sudoku logic, and cross-hatching applies it in its purest form. Run this pattern for every digit in every box and you'll crack most Easy puzzles wide open.

Technique 3: Pencil Marks (Notes)

When scanning and cross-hatching stop producing answers, it's time to write down what you know. Pencil marks (also called candidates or notes) are the small digits you jot in a cell's corner listing everything that cell could be.

When to start marking

Don't mark up the whole grid on move one — that's 500+ tiny digits of noise. The sweet spot: start penciling when the easy placements dry up, usually when 20–30 cells are filled. Mark the cells in the emptiest regions first, since those need the tracking most.

Full notation vs. quick marks

On SudokuPuzzle.pro, notes mode lets you toggle pencil marks per cell with a tap, so experimenting costs you nothing.

Reading your marks: the sole candidate

Once marks are down, scan them for the sole candidate — a cell with only one pencil mark left. That digit is forced; fill it in. Then update the marks around it (remove that digit from its row, column, and box), and look for the next sole candidate. Chains of these will finish many Medium puzzles.

Putting It All Together: A Solving Rhythm

Here's a simple loop that works on every puzzle up to Medium:

  1. Sweep — do a 1-through-9 inventory scan; fill everything obvious.
  2. Cross-hatch — target each digit in each box; fill what you find.
  3. Mark — pencil the remaining empty cells (full notation for Medium).
  4. Harvest — fill every sole candidate, update marks, repeat until stuck.
  5. Loop back — new placements unlock new scans. Return to step 1.

When even this loop stalls, you've officially graduated to intermediate strategies: hidden singles, pairs, and pointing patterns that squeeze logic out of your pencil marks.

Practice these techniques free on SudokuPuzzle.pro — try them on Easy first, then take on the Daily Challenge (a fresh Hard puzzle every day) to build your streak.

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