Why You Miss Candidates: Scanning Three Rows at a Time

The Symptom: The 'Invisible' Candidate

You know the feeling. The puzzle seems to grind to a halt. You've filled in all the obvious numbers, but now every empty cell has two or three tiny penciled candidates. You check and re-check your rows and columns, but nothing clicks. So you press the Hint button, and it highlights a cell with a single candidate—a digit you never even wrote down. It was a valid placement all along, hidden in plain sight.

This isn't a failure of logic; it's a failure of observation. The symptom is a recurring dead stop where the solution requires spotting a digit that can only go in one cell within a row, column, or box. Often, this missed opportunity is a Hidden Single within a box. The frustration comes from knowing the rules but not seeing the board correctly.

• If you frequently use the 'Check' or 'Hint' function to reveal a candidate you missed, this is the primary symptom.
• A feeling of 'I've looked at that box ten times!' is a strong indicator your scanning pattern has a gap.

The Cause: Row-by-Row Scanning Creates Box Blind Spots

The root cause is a natural but flawed scanning habit: reading the puzzle like a book, one horizontal row from left to right. You look at Row 1, then Row 2, then Row 3. While this feels thorough for checking rows, it fragments your view of the 3x3 boxes. A box is a unit defined by three rows and three columns. When you scan only one of its rows at a time, you cannot see digit patterns that span all three.

Consider a digit '5'. To find all its possible placements in the top band (boxes 1, 2, and 3), you need to see where '5' is already placed and where it is blocked across Rows 1, 2, and 3 *simultaneously*. Row-by-row scanning forces you to hold this cross-box information in your memory, which is prone to error. A candidate might be eliminated from a box because of a placement in a different row of the same band—a connection you'll miss if you don't view the band as a whole. This blind spot is precisely what techniques like Box-Line Reduction and Pointing Pairs formalize; they require seeing the interaction between a line (row/column) and a box. Your inefficient scanning method makes these interactions invisible.

• Diagnosis Checklist: 1) Have you placed every digit (1-9) in all nine boxes? 2) For each digit, have you checked its possibilities across all three rows of a horizontal band before moving to the next band?

The Fix: Scan by Digit Across Three-Row Bands

The fix is to change your fundamental scan unit from a single row to a three-row horizontal band. Work systematically, one digit at a time, across the entire grid. This method aligns your observation with the puzzle's core structure: boxes and bands.

Here is the step-by-step routine: 1. **Pick a Digit:** Start with digit 1. 2. **Scan a Band:** Look at the top band (Rows 1-3). Mentally note every cell already containing a '1' in these three rows. Then, for each of the three boxes in this band, determine if a '1' is already placed inside it. If not, identify all possible cells for '1' in that box, considering blocks from the rows *and* from other '1's in the band. 3. **Look for Opportunities:** As you do this, you will immediately see if a '1' can only go in one cell in a box (a Hidden Single) or if its candidates are restricted to one row/column within a box (creating a Pointing Pair for elimination). 4. **Repeat:** Complete this scan for digit '1' across the middle band (Rows 4-6) and then the bottom band (Rows 7-9). 5. **Cycle Through Digits:** Move to digit 2 and repeat the entire process. Continue through all digits from 1 to 9.

This 'digit-by-digit, band-by-band' scan ensures you never miss a box-level constraint. It forces you to integrate information across three rows, eliminating the blind spot. After a full digit cycle, switch to scanning by column bands (three vertical columns) to catch constraints you might have missed from the horizontal perspective. For more on building efficient habits, see our guide on How to Solve Sudoku Quickly.

Practice Routine to Build the Habit

Old habits are hard to break. To cement this new scanning method, dedicate your next five puzzle sessions to deliberate practice. Start with Easy or Medium puzzles where the pressure is lower.

**Focused Practice Steps:** 1. **Pencil Marks Second:** For the first few puzzles, do not write any pencil marks (candidates) until you have completed at least two full digit scans (e.g., digits 1-9 horizontally, then 1-9 vertically). This trains your brain to see placements without the crutch of notes. 2. **Verbalize Your Scan:** Silently talk yourself through it. 'Scanning for 5. Top band: a 5 in R2C4, so not in Box 2. Box 1 has a possible 5 only in R3C2... found a Hidden Single.' 3. **Band Completion:** Make a rule: you cannot leave a band while scanning a digit until you are confident you've found every possible placement and elimination for that digit within those three boxes. 4. **Review with Hints:** After you feel stuck, use the Hint button. If it reveals a basic Hidden Single or Pointing Pair, analyze *why* your scan missed it. Was it because you didn't consider all three rows of the band together?

• Use the 'Check Possibilities' feature on our site *after* your scan to see if it matches the engine's full candidate list. This provides immediate feedback.
• Time yourself. Initially, this method may feel slower, but as it becomes automatic, your overall solve speed will increase due to fewer dead stops.