How to Spot Hidden Singles in Sudoku

By Avi Israeli · Reviewed by Ninely Engine Logic Council · Updated November 2026

What is a Hidden Single?

A Hidden Single occurs when a particular digit can only fit in one remaining cell within a house (a row, a column, or a 3×3 box), even though that cell still has multiple candidate numbers open.

It is called "hidden" because if you inspect only that single cell, you see several legal pencil marks—such as {2, 3, 8}—so it is not a Naked Single. However, when you scan the entire 3×3 box, you discover that 2 is blocked from every other empty cell in that box by intersecting rows and columns!

Worked Example: Placing 2 in Box 4 (Row 6, Column 2)

Examine the middle-left 3×3 box (Box 4, spanning Rows 4–6 and Columns 1–3) on this Ninely practice board. Box 4 already contains 6, 9, 1 and needs a 2:

  • Row 5 (R5C1, R5C2): Eliminated by the given 2 at Row 5, Column 6 (R5C6).
  • Column 3 (R4C3, R6C3): Eliminated by the given 2 at Row 7, Column 3 (R7C3).
  • Column 1 (R6C1): Eliminated by the given 2 at Row 3, Column 1 (R3C1).

Even though cell Row 6, Column 2 (R6C2) has three candidate pencil marks {2, 3, 8} (since its peers contain 1, 4, 5, 6, 7, 9), it is the only cell in Box 4 where 2 can legally live!

Before (Candidates in R6C2)
23 8
➔
After (Hidden Single in Box 4)
2
7
9
2
5
2
7
1
2
1
7
6
9
1
8
1
3
2
8
2
6
5
9
7
2
8
9
4
1
7
3
9
5
4
Step 3 of 3: Digit 2 locked into R6C2

⚠️ The Tempting Wrong Move: Guessing 8 at R6C2

Because cell R6C2 has candidates {2, 3, 8} and Row 6 is missing 8, a player looking only at Row 6 might guess 8 in R6C2.

Why this fails: Filling R6C2 with 8 leaves Box 4 with zero legal cells for 2 (since R3C1, R5C6, and R7C3 block every other empty square in Box 4)! Always check whether a house requires a specific digit before guessing among a cell's pencil marks.

💡 Solving Tip: Cross-Hatching Technique

The fastest way to spot Hidden Singles in 3×3 boxes is cross-hatching: pick one digit (such as 2), trace horizontal and vertical laser lines from existing 2s across neighbouring boxes, and see which box is narrowed down to a single unblocked cell.

Ready to Spot Hidden Singles?

Test your cross-hatching speed on today's free daily puzzle:

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