Expert 16x16 Sudoku — Chains Across 256 Cells

Expert 16x16 sudoku is rated 5.0 to 6.9. Fish and wings are not enough here, and chains of linked candidates decide the puzzle. It suits solvers who already finish hard 16x16 grids and want longer deductions.

What this level asks

Tracking. A chain on 16x16 can pass through many more cells than on 9x9, and a single missing candidate can hide the link you need. Most of the solve goes on notes and on checking them. If a chain fails, a stale note is the likeliest cause.

Techniques you will meet

• Jellyfish (5.2): a four-line fish, where a symbol confined to the same four columns across four rows clears the rest of those columns.
• Unique rectangles (4.5 to 5.0): uses the fact that a valid puzzle has one solution to rule out a deadly pattern of four cells.
• X-cycles (from 6.5): a loop of links on one symbol that forces eliminations.
• XY-chain (from 6.6): a chain of two-candidate cells where each hands the next its value.

Chains are built from links, and every link is a statement such as "if this cell is B then that cell is not B". On a bigger board there are more links to choose from, which makes chains easier to find but longer to verify. Start from a two-candidate cell, follow one branch until it forces something, and confirm each step against the notes before trusting the elimination at the end. Unique rectangles are the quick exception: they need only four cells.

Tips for this level

• Look for cells with exactly two candidates first. Chains start there.
• Re-check notes after every placement.
• Take a break when stuck. The board is saved after each move.

Next step

Practise the same tools on a smaller board at expert 9x9, or go on to master 16x16.