Guides

Which Sudoku technique should I use next?

Getting stuck does not mean that a Sudoku needs a guess. It usually means that the next useful observation has not been named yet. This guide is a decision path, not a catalogue: start with the cheapest check, take one logical action, then return to the top. The aim is to choose a technique because the grid proves it applies—not because its name sounds advanced.

First, make the current state trustworthy

Before looking for a pattern, check the last move and the three affected units: its row, column and box. Then scan units with only one or two empty cells. A missed digit or an outdated candidate can make every later technique look impossible. If you use notes, remove candidates contradicted by the newest placed digit before searching further. A clean grid turns a vague stall into a question you can answer.

Do you have a single right now?

Ask two short questions. Does one empty cell have only one candidate left? That is a Naked Single. Or can one digit go in only one cell of a row, column or box? That is a Hidden Single. Place the proven digit, update the affected candidates and scan again. Singles often appear in a chain; learning to notice that chain is more valuable than jumping immediately to a complex pattern.

If there is no single, compare candidates within one unit

Choose a row, column or box with a manageable number of empty cells. Look for two cells containing the same two candidates: a Naked Pair. Those two digits cannot occur anywhere else in that unit, so remove them from the other notes there. Then reverse the view: if a digit appears in only two cells of one unit, check whether both cells align in a row or column. This keeps the search local and prevents random note-editing.

Check the boundary between a box and a line

When a candidate in one three-by-three box is restricted to a single row or column, it cannot appear elsewhere on that line. This is a Pointing Pair. The reverse direction is called Claiming: if all candidates for a digit in a row lie in one box, remove that digit from the other cells in the box. Both moves use the same proof—one digit is confined to two overlapping units—and are a strong next step on medium and hard puzzles.

Only then widen the search

If singles, pairs and locked candidates are absent, widen your view one digit at a time. Check whether its candidates form a simple rectangle across two rows and two columns (X-Wing), then consider wings or chains only when their exact condition is visible. Do not hunt every advanced name at once. Pick one digit, state the condition aloud or in a note, and make only the eliminations that the pattern proves. If the condition is incomplete, return to the smaller checks instead.

Turn the next stall into a known next step

Open the technique library for worked, clickable proofs, or take a hard puzzle and follow this decision order without guessing. Start with one exact lesson, not a long list of patterns.

Techniques Play Hard Solver

Naked Single Naked Pair Pointing Pair

Frequently asked questions

Should I always search for advanced techniques when I am stuck?

No. Recheck singles, candidates and locked candidates first. They are quicker to verify and often appear after one correct placement. Advanced techniques are useful when their exact condition is present, not as a default search.

What is the fastest way to choose a technique?

Use a fixed order: clean the grid, scan for singles, compare candidates in one unit, check box-line interactions, then inspect one digit for an advanced pattern. A fixed order reduces repeated scanning and keeps every move explainable.

Do I need candidate notes for every hard Sudoku?

Not always, but they make pairs, locked candidates and advanced patterns visible. Keep them current. Incorrect or stale notes are worse than no notes because they hide the proof you are looking for.

When should I use a hint?

Use a hint after you can say which branch of the decision path failed. Read the explanation, identify the same condition on the board, and then open the related lesson or exercise. A hint should reveal a proof, not replace it.