Lesson 01 of 08 — Beginner
Single candidate technique
A single candidate is a cell with one digit left to try. Every other digit already appears somewhere in the row, the column or the box, so the cell has only one possible value. It is the simplest technique of the eight, and it is where every puzzle starts.
What it does
Take one empty cell and look at the twenty cells that share its row, its column or its box. Between them they hold most of the digits from one to nine, and none of those can go in your cell.
Cross off everything you can see, and count what survives. One digit left is a single candidate, and it is certain: no arrangement of the rest of the grid can put anything else there.
A worked example
Choose a crowded cell
Start where the grid is fullest. A cell in a row, a column and a box that are each nearly finished has the fewest digits left to consider, so it is the quickest to check.
Read the three units
Write down what the row, the column and the box already contain. Overlaps do not matter; you are only asking which digits from one to nine are already spent.
Count what is left
If one digit appears in none of the three, the cell is that digit. If two or more are missing, the cell needs a different technique, so leave it and move on.
Place it and look again
Writing the digit in removes it from every cell that can see it. That often leaves a neighbouring cell with a single candidate of its own, so repeat the scan around the digit you just placed before moving elsewhere.
How to spot it
- Rows, columns and boxes that are nearly full, and cells with many filled neighbours.
- A puzzle that has just had several digits placed in one area: the cells beside them are the ones that have lost the most candidates.
- Your own pencil marks. A cell showing a single mark is a single candidate, but only if you wrote the marks correctly.
A single candidate is only as good as your notes. Miss one digit when writing the candidates and the cell will look finished when it is not, and every later scan that trusted it is wrong. The technique also cannot help while a cell still has two or more candidates: look for a hidden single or a locked candidate first.
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