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Before diving into the techniques themselves, it helps to know the three phases every puzzle goes through, whatever its level:

• Scanning — you sweep the grid with your eyes to place the obvious digits, without writing anything down.
• Candidates — once direct scanning isn't enough anymore, you pencil in every number still possible ("candidates") in each empty cell.
• Analysis — you eliminate candidates one by one, applying increasingly refined techniques, until a cell is left with only one candidate.

These three phases loop back on each other: every digit placed or candidate eliminated can reveal a new one elsewhere in the grid. It's this iteration that does all the work — never a random guess.

Easy only requires a single technique: the Hidden Single. For a given number, only one cell remains possible in a row, column or block — you place it directly, no notes needed.

Medium adds spotting Locked Candidates: if a number can only go in a single row (or column) within a block, it can be eliminated from the rest of that row or column elsewhere in the grid — and vice versa. Notes aren't required yet at this level, but they quickly become useful to avoid mistakes.

Details and examples: Solving and Region interactions on the How to play page.

From Hard on, direct scanning is no longer enough: you need to note all candidates in every empty cell. Four technique families are added:

• Naked Single — a cell has only one noted candidate left: that's the answer.
• Twins, Triples and Isolated groups — N cells in a unit collectively contain only N candidates: those numbers are eliminated from every other cell in the unit.
• Hidden Pairs and Triples — N numbers only appear as candidates in N cells of a unit: all their other candidates are eliminated.
• Mixed groups — the extended version of hidden pairs/triples, up to 4 numbers confined to 4 cells.

The detail of each, with a worked example, is in the How to play guide.

Expert introduces four techniques that rely on a visual pattern to spot in the grid — a rectangle, a wing — rather than a long chain of deductions:

• X-Wing — a candidate aligned in a rectangle across 2 rows × 2 columns is eliminated from the rest of those columns (or rows). Our article The X-Wing, Explained Simply covers it in detail, diagram included.
• Skyscraper — an "asymmetric" X-Wing with 4 cells, sharing only a single column (or row).
• Swordfish — the extension of the X-Wing to 3 rows × 3 columns.
• XY-Wing — a pivot cell with 2 candidates and two branches that let you eliminate a shared candidate elsewhere in the grid.

The most demanding level, Evil, requires building and following a chain of deductions across the whole grid — a different mental exercise from the local patterns of the Expert level, not just "one notch harder":

• Simple Coloring — you alternately color cells linked by strong links to deduce a value by contradiction or convergence.
• XY-Chains — the extension of the XY-Wing to a chain of arbitrary length.
• X-Chains — a chain of alternating deductions (strong/weak links) involving a single candidate number.

When a puzzle resists, here's the order to follow — always from the least to the most mentally costly:

1. Scan the grid for Hidden Singles — it's free, no notes required.
2. As soon as you get stuck, note every candidate in each empty cell.
3. Look for Locked Candidates, then twins/triples and hidden groups — from simplest to rarest.
4. If the puzzle still resists, move on to the Expert-level patterns (X-Wing, Skyscraper, Swordfish, XY-Wing).
5. As a last resort, on Evil puzzles, build a chain (Simple Coloring, XY-Chains, X-Chains).

After every elimination or digit placed, start back at the top of this list: an advanced technique often reveals a plain Hidden Single somewhere else in the grid.

Every technique listed here has its own full explanation, with a concrete example, in the How to play guide. For the exact mapping between difficulty level and required techniques, head to Techniques by level. And to go deep on one technique in particular, check our dedicated articles — starting with The X-Wing, explained simply, diagram included.