Hidden Singles
Learn the hidden single: a digit that can legally go in only one cell of a row, column or box, even when that cell has other candidates.
A hidden single is a digit that fits in only one cell of a unit — a row, column or box — even though that cell may still show other candidates. The digit is "hidden" among them, but it has nowhere else to live in the unit.
You find hidden singles by scanning a single number at a time: pick a digit, and in each box ask "which cells could hold it?" If only one can, that is a hidden single.
This is the technique that takes you from easy to genuinely fast solving. The highlighted unit in the example below contains a digit that fits only one of its cells.
Practise the Hidden single
The best way to learn a technique is to use it. Play a puzzle at the level where it first appears, or drop a tricky board into the solver to watch it in action.
Frequently asked questions
How do I find hidden singles?
Scan one digit at a time through each row, column and box. Wherever that digit can legally go in only a single cell of the unit, you have a hidden single.
Are hidden singles harder than naked singles?
Slightly — they take active scanning rather than just reading a cell — but they are still a beginner technique and appear in every puzzle.
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