How to Solve Master and Evil Sudoku Puzzles

The Foundation: Confirming the Basics Are Exhausted

Before tackling advanced patterns, you must be certain all basic techniques are fully applied. This isn't just about finding an obvious Naked Single. It means repeatedly scanning the grid to ensure no Hidden Singles, Naked Pairs/Triples, or Hidden Pairs/Triples remain. On Evil puzzles, these often appear only after several rounds of candidate elimination from more advanced steps. A common trap is to assume basics are done after one look; true mastery requires cycling back to them constantly as the puzzle state changes.

• Use pencil marks for all candidates. It's impossible to spot advanced patterns without them on Master+ puzzles.
• After every elimination from an advanced technique, immediately re-scan for new Hidden Singles or pairs.

The First Advanced Rung: Locked Candidates (Pointing & Claiming)

The first step beyond pairs is spotting Locked Candidates, also known as Pointing Pairs/Triples and Claiming. A Pointing Pair occurs when all instances of a candidate in a 3x3 box are aligned in a single row or column. This 'points' outward, allowing you to eliminate that candidate from the rest of that row or column outside the box. Conversely, a Claiming pattern happens when all instances of a candidate in a row or column are confined to one box, letting you eliminate it from the rest of that box. These techniques are gatekeepers; if you miss them, more complex patterns like X-Wing become much harder to spot.

• Scan each number 1-9 separately. Look at each box: are all candidates in one row/column? Look at each row: are all candidates in one box?

The Intermediate Leap: Fish Patterns (X-Wing, Swordfish)

When Locked Candidates are exhausted, the puzzle typically demands 'Fish' patterns. These are size-based: a 2-row, 2-column pattern is an X-Wing. If a candidate appears exactly twice in two rows and those positions line up in two columns (forming a rectangle), you can eliminate that candidate from the rest of those two columns. Swordfish is the 3-row, 3-column version. The mental trap is looking for perfect rectangles; focus instead on the definition: a candidate must be a potential in exactly N rows, and all those potentials fall into exactly N columns (or vice-versa). Successfully applying Fish patterns is a hallmark of solid sudoku strategies for hard puzzles.

The Advanced Rung: Chain Logic (Y-Wing, AIC)

For the toughest Evil puzzles, you need chain logic. A Y-Wing is a powerful three-cell chain that works like a hinge. If cell A shares candidates with cells B and C, and B and C share a candidate not in A, then that shared candidate can be eliminated from any cell seen by both B and C. Beyond Y-Wings lie Alternating Inference Chains (AIC), the true workhorse of expert solving. An AIC uses alternating 'if this, then NOT that' links between candidates. If the chain starts and ends with the same digit in different cells, you can perform eliminations. Spotting these requires looking for strong links (a candidate appears only twice in a unit) and connecting them.

• To find a Y-Wing, look for a bi-value cell (with only two candidates). See if its two 'wings' see two other cells that share a common candidate.
• For AIC, practice by looking for strong links for a specific digit and seeing if they can be connected via other candidate relationships.

Putting It All Together: The Systematic Scan

The strategy is not just knowing techniques, but applying them in a systematic order. 1) Fill all pencil marks. 2) Exhaust all basic techniques (Singles, Pairs, Triples). 3) Scan for Locked Candidates (Pointing/Claiming). 4) If stuck, methodically search for Fish (X-Wing, then Swordfish). 5) Finally, look for chain patterns like Y-Wing and AIC. After any successful elimination from steps 3-5, always loop back to step 2. The common failure point is jumping to chains before confirming no Fish exist, or giving up before a well-executed AIC cracks the puzzle wide open.