Expert Sudoku — Chains and Advanced Fish

Expert Sudoku is rated 5.0 to 6.9 on the Sudoku Explainer scale (ER), and a puzzle can sit still for minutes here. Our classic expert grids carry 21 to 29 clues, 24.7 on average, so the grid is sparse and nearly every empty cell starts with several candidates. It suits solvers who already know the standard toolkit and want more.

What this level asks

Standard patterns clear part of the grid, then you need a longer line of reasoning. A single deduction can depend on three or four linked cells. Accurate pencil marks are not optional, and the best habit is the stall loop: when stuck, try the simplest technique you have not tried yet, not the fanciest one you know.

Techniques you will meet

The diabolical tier lives here. Unique rectangles span 4.5 to 5.0 and reason about the puzzle itself: a valid sudoku has exactly one solution, so an arrangement that would give two cannot be right. Jellyfish rates 5.2 and is an X-Wing widened to four lines. WXYZ-Wing rates 5.5 to 5.6 and Aligned Pair Exclusion 6.2. Near the top, X-Cycles and XY-Chains start at 6.5 and 6.6, growing with chain length. An XY-Chain is a row of two-candidate cells with the same digit at both ends, so any cell seeing both ends can drop it.

Tips for this level

• Colour or tag cells on paper if a chain is longer than four.
• Update notes after every placement, since a stale candidate breaks a chain.
• Take a hint when a chain will not close, then study which technique it used.
• Use undo freely. Every grid has exactly one solution, so logic always wins over guesswork.

Next step

Chains feeling natural? Master Sudoku groups and nets them. Stuck? Step back to Hard.