Hidden Single
A digit fits only one cell within a unit.
A hidden single is a digit that has just one legal home inside a house — here a 3x3 box — even though that cell still holds other candidates. Hidden single sudoku solving works from the house down to the cell: you take one digit the box is missing, scan the box, and find only one square where it can still go. It is the most common way a box gets filled once the obvious naked singles are gone.
Worked example
Box 1 already holds the givens 1, 2, 4, 5, 6 and 8, so it still needs a 3, a 7 and a 9 across its three empties R1C3, R2C2 and R3C3. The 7 sitting lower at R7C3 shares column 3 with both R1C3 and R3C3, striking 7 out of each, which leaves R2C2 as the only cell in box 1 that can take a 7 — so 7 is placed at R2C2. Note that R2C2 is not a naked single (a 3 or a 9 would still fit there), yet 7 is hidden as the box's sole home for it.
When to use it
Reach for a box hidden single right after basic scanning and before you commit to a full grid of pencil marks — it is usually the fastest legitimate placement available. To spot it, pick a digit the box still needs, then test each empty cell against that digit's appearances in the rows and columns cutting through the box; if every empty but one is knocked out, that survivor takes the digit. A line-crossing clue like the 7 in column 3 is the tell: one digit outside the box quietly deleting it from all but one of the box's open cells.
How to use it
- Pick a digit and a unit. Choose a digit (say 4) and a single row, column, or box that does not yet contain it.
- Test each empty cell. Mark which empty cells in that unit could legally hold the digit, ruling out cells whose peers already use it.
- Only one fits. If just one cell can take the digit, place it there — even though the cell has other candidates too.
Frequently asked questions
What is a hidden single in Sudoku?
A hidden single is a digit that can legally go in only one cell of a given box, row or column, even though that cell still shows other candidates. It differs from a naked single, where the cell itself has only one candidate remaining. In the box version you place the digit because it has nowhere else to live inside that box — as with 7, which can only sit at R2C2 in box 1.
How do I spot a hidden single in a box?
Take a digit the box still needs and look at where that digit already appears in the rows and columns passing through the box. Cross off every empty cell those clues can see; if a single empty cell survives, that is a hidden single and the digit goes there. In the worked board the 7 at R7C3 removes 7 from R1C3 and R3C3 down column 3, leaving only R2C2.
Why is it called 'hidden' if the cell still has other candidates?
It is hidden because the cell looks ambiguous on its own — R2C2 could still be a 3 or a 9 — so nothing about the cell by itself points to 7. The single only becomes visible when you scan the whole box for the 7 and find it has just one legal spot. The cell's other candidates are irrelevant: a digit with one home in its box must be placed there.