Master Sudoku — Forcing Chains and Nets

Master Sudoku is our extreme sudoku level, rated 7.0 to 9.9 on the Sudoku Explainer scale (ER). Our classic master grids have 19 to 28 given digits, 24.4 on average, fewer than any other level, so the board opens with many candidates per cell. The jump from Expert is not more of the same: single chains give way to reasoning about groups of cells.

What this level asks

Plan for several deep deductions in one puzzle. You will often follow two or three branches of a forcing chain before an elimination holds. Finishing one takes patience rather than speed, often over an hour, and it suits solvers who enjoy the reasoning itself, not just the finished grid.

Techniques you will meet

This is the extreme tier. Alternating inference chains start at 6.5 and lengthen with each link. They are the general form of every chain: you hop between digits and cells, alternating two kinds of link, and read off what must be true at the ends. Finned X-Wing is an X-Wing that almost works, with an extra candidate (the fin) confined to one box. Almost locked sets extend the set ideas from Hard and Expert. Forcing nets begin at 8.5 in Sudoku Explainer, so they appear in the upper part of this band. You try every candidate of one cell and keep what all branches agree on.

Tips for this level

• Write down each branch's results before comparing them. Memory fails at depth.
• Test the cell with the fewest candidates first, since short branches finish fastest.
• Ask for a hint when a branch will not resolve, then study which technique it used.
• Walk away freely. The board autosaves after every move, so you can leave a puzzle overnight.

Next step

When Master feels steady, Legendary Sudoku is the final step.