"Hidden Singles" Technique
What is a "Hidden Single"?
"Hidden singles" is a quite simple Sudoku technique, but it requires careful attention from the player because it can be hard to spot.
The point of "Hidden singles" is that a Note (candidate) is the only one of its kind in an entire row, column, or 3x3 block, even though the cell itself contains other notes.
"Hidden" vs. "Obvious" Singles
- An "Obvious Single" (Naked Single) is a cell with only one note (e.g., {2}).
- A "Hidden Single" is a cell with multiple notes (e.g., {1, 7, 8}), but one of those notes (e.g., the '1') appears nowhere else in that block's notes.
How to Find a Hidden Single
It will be easier to understand this technique by looking at the example in the top-right 3x3 block.
In this block, we can see the following filled numbers and notes:
- Filled Numbers: 5 (R1C7), 6 (R2C7), 2 (R2C8).
- Notes (Candidates):
R1C8 = {3, 4, 9}
R1C9 = {3, 4, 9}
R2C9 = {3, 4, 7, 8}
R3C7 = {1, 7, 8} (Our highlighted cell)
R3C8 = {7, 8, 9}
R3C9 = {7, 8, 9}
Step 1: Scan the Notes for a Single Number
Instead of looking at a single cell, we must scan the entire block for a specific number. Let's hunt for the number 1.
R1C8? No '1' note.
R1C9? No '1' note.
R2C9? No '1' note.
R3C7? Yes, it has a '1' note.
R3C8? No '1' note.
R3C9? No '1' note.
Step 2: Find the Solution
The number 1 appears as a candidate in only one cell in this entire 3x3 block: cell R3C7.
Even though R3C7 also contains notes for 7 and 8, it is the only possible place the 1 can go in this block. Therefore, R3C7 must be 1.
We can now remove all the notes from this cell and put the number 1 in its place.
Master This Scanning Technique
That's it for the "Hidden singles" technique. Once you have learned it, you can proceed with the following Sudoku strategies.
Ready to Practice Hidden Singles?
The best way to master Hidden Singles is to **find them yourself in a real puzzle**.
Start with an Easy or Medium puzzle, turn on Notes, and scan each row, column, and 3×3 block for numbers that appear only once as a candidate.
Tip:If you can spot Hidden Singles without checking every cell one by one, your scanning skill is already improving.