Lesson 02 of 08 — Beginner
Hidden single technique
A hidden single is a digit with one home left. The cell holding it still looks busy, with two or three candidates written in it, but the digit you are chasing can go nowhere else in that row, column or box. So the cell must be that digit, and its other candidates are wrong.
What it does
The single candidate reads one cell and counts digits. The hidden single turns that around: pick one digit, and count the cells in a unit that can still take it.
If exactly one cell in the unit allows the digit, that cell is the digit. Nothing else has to be decided first, and no other candidate in that cell can be true.
A worked example
Pick a unit
Choose a row, a column or a box, and prefer one that already holds a lot of digits. The more of the unit is filled, the fewer cells you have to check.
Pick a digit
Take one digit that is not placed in that unit yet. You are asking a question about that digit alone, not about any particular cell.
Count its homes
Go through the empty cells of the unit and ask of each one whether the digit is already in its row, its column or its box. Any cell where it is already present is ruled out.
One home means placed
If a single cell survives, write the digit into it and clear the other candidates from that cell. If two or three survive, there is no hidden single here for that digit; move to the next digit or the next unit.
How to spot it
- Work digit by digit rather than cell by cell. The pattern is invisible from a single cell, which is exactly what makes it "hidden".
- Boxes and rows that are missing just one or two digits are the quickest places to find one.
- After a placement, re-check the digit you placed in the other two units that cross it: one placement removes three homes at once.
The count has to cover the whole unit. A cell that allows three different digits can still be a hidden single for one of them, so do not skip a cell because it looks busy. And if your candidate list is incomplete, the count will say two homes where there is only one, and the move will be missed rather than made wrongly.
Practise it in Sudoku Mori
The game teaches every technique in this order, each with a worked example and three puzzles of its own. Lesson puzzles are kept apart from the daily ones, and any lesson can be revisited.
Keep reading
All eight: Single candidate · Hidden single · Locked candidates · Naked pair · Hidden pair · Naked triple · Hidden triple · X-Wing