Master 16x16 Sudoku — Forcing Chains, Giant Size

Master 16x16 sudoku is rated 7.0 to 9.9. The extreme tier takes over: alternating inference chains, almost locked sets and forcing nets. It suits experienced solvers who read chains comfortably and have time for a marathon. Several of these techniques may be needed in one puzzle, and each must be followed across a 256-cell grid.

What this level asks

Patience and a clean method. The difficulty levels guide explains how the bands work. Master grids take much longer than their 9x9 equivalents because each deduction involves more cells, and a wrong assumption can run a long way before it breaks.

Techniques you will meet

• Alternating inference chains (from 6.5): links that alternate between "this is false" and "this is true" until the ends prove an elimination.
• Almost locked sets: a group of cells holding one candidate more than it has cells, used as a link between houses.
• Forcing nets (from 8.5): branching chains where every branch of an assumption reaches the same conclusion.

Expect to spend most of the time on the board's candidates rather than on placing symbols. A chain that works on paper can still fail if one cell on the path has an extra candidate you forgot, so rebuild the notes whenever a result looks too good. If you are stuck, check for a simpler tool first: a pointing pair or a swordfish hiding in the notes often saves a whole chain. Progress is saved after every move, so a break costs nothing.

Tips for this level

• Write chains down as you build them, one link per line, so you can retrace a wrong turn.
• Prefer short chains over clever ones.
• Treat the puzzle as several sittings, not one.

Next step

If chains are not yet comfortable, return to expert 16x16. Ready for the top? Try legendary 16x16.