Sudoku strategy and techniques

Five techniques, in the order you will need them

Sudoku technique has an enormous vocabulary and almost none of it is necessary. The five below solve every fair board up to expert level. They are ordered by when you first need them — not by how clever they sound.

1. Single candidate

A cell that sees eight different digits across its row, column and box has one possible answer. Nothing else fits.

This is the whole of easy: boards on that level are calibrated so single candidates alone finish them.

2. Hidden single

Look at a unit — one row, column or box — and ask where a particular digit can go. If a 7 fits in only one cell of that box, that cell holds the 7, even if it also had five other candidates.

The mental switch here is the important part: you stop asking what fits in this cell and start asking where does this digit fit. Most people find hidden singles harder to see than single candidates, though they are logically no deeper.

3. Locked candidates

If every possible place for a digit inside a box lies in one row, that digit cannot appear in the rest of that row outside the box. And the other way round: if every place for a digit in a row lies inside one box, it cannot appear elsewhere in that box.

You write nothing down — you only strike candidates out. That is the moment pencil marks stop being optional, and the moment medium becomes a different game from easy.

4. Naked pair and hidden pair

• Naked pair — two cells in one unit hold exactly the same two candidates. Those two digits are reserved for those cells, so strike them from every other cell in that unit.
• Hidden pair — two digits that fit only in the same two cells of a unit. Those cells then hold nothing else, however many candidates you had written in them.

Pairs are the backbone of hard. When a board looks like it needs something exotic, it is almost always a missed pair.

5. Naked triple

Three cells that between them use exactly three candidates. They do not each need all three — the union being three is enough. Strike those three digits from the rest of the unit.

With this the toolbox is complete for every board on this site, including expert.

What you can safely skip

X-Wing, swordfish, XY-chains, forcing chains — real techniques, all of them, and none required here. Our generator accepts a board requiring X-Wing only if one turns up by itself, because boards that genuinely need it are rare and hunting for them costs seconds of CPU for no benefit to the player. For a human racing the clock, expert is hard because of how few digits it starts with, not because of a technique with a name.

Three habits worth more than any technique

1. Read the grid digit by digit, not cell by cell. Take one digit and find all its places. This single change cuts most people's time more than learning a new technique.
2. Never write a digit you cannot justify. Put doubts in pencil marks — a mark costs nothing, a wrong digit costs a life.
3. Review your marks after every breakthrough, in the row, column and box you touched. A stale pencil mark is the most common source of mistakes at every level.

Ready to use them? Start on medium — it is the level where technique first pays off. If the terms above were new, the rules page covers what comes before them.