Expert sudoku

What makes a puzzle expert

An expert puzzle needs at least one technique from the top of our list: Swordfish, Jellyfish, XYZ-Wing, a unique rectangle (type 1) or simple coloring. Across our expert bank, 80% of puzzles use simple coloring, 23% an XYZ-Wing, 15% a unique rectangle and 4% a Swordfish. Jellyfish are rare enough to round to 0%. Expert puzzles also use everything below them: 29% include an XY-Wing and 55% a naked pair.

What expert puzzles never need is guessing. Our grader rejects any puzzle that needs chains, forcing nets or trial and error, so a careful solver can always find the next step by logic.

The expert techniques in brief

• Swordfish and Jellyfish extend the X-Wing to three or four rows and columns: if a digit's possible cells in three rows all fall in the same three columns, it can be removed from the rest of those columns.
• XYZ-Wing: like the XY-Wing, but the pivot has three candidates. Cells that see the pivot and both pincers cannot hold the shared digit.
• Unique rectangle (type 1): four cells in two rows, two columns and two boxes. If three of them hold only the same pair, the fourth cannot be either of those digits, or the puzzle would have two solutions.
• Simple coloring: follow a digit through rows, columns and boxes where it has exactly two possible cells, alternating two colours. If two cells of one colour see each other, that colour is false; a cell that sees both colours cannot hold the digit.

How to approach an expert sudoku

Patience and tidy notes. Take every single, fill in full candidates and apply the medium and hard techniques before looking for anything exotic. Many expert puzzles need only one expert step in the middle; after it, the rest falls to simpler logic. When you are stuck, pick one digit and look at where it can go across the whole grid: X-Wings, Swordfish and coloring all show up when you study a single digit at a time.

The timer pauses when you leave the page, and your progress is saved, so an expert puzzle can be a project over several sittings. If you want to check your reasoning afterwards, paste the starting grid into the solver to see one logical path through it.

Too hard, or not hard enough?

If expert feels like a wall, spend some time on hard puzzles and read the strategy guides; the expert patterns build directly on X-Wings and XY-Wings. The daily sudoku also has an expert level, the same for everyone, so you can compare notes.