Hard Killer Sudoku — Combinations and Innies

Hard Killer Sudoku is rated 3.0 to 4.9, the tough tier, and starts with only a few given digits. Reading one cage at a time stops working here. You will relate cages to each other and to the units they overlap. It suits players who treat combinations and one-cell innies as routine.

What this level asks

A single cage rarely settles a cell any more. A pair of cages, or a cage against a row, column or box, usually does. Hard classic puzzles ask for patterns such as X-Wing, and hard killers add the arithmetic versions on top. Notes matter from the first minute, because a cage that looks free often has one digit already excluded by a neighbour. Expect a solve of real length, but each step is a short, checkable inference.

7

Possible combinations: 1

• 124

Techniques you will meet

• Cage splitting (project rating 3.2): separate the cells of a cage that lie inside one unit from those outside it.
• Multi-cell innies and outies (3.4): the 45 rule applied to groups of cages.
• Hard killer combinations (2.5): sums with several digit sets, narrowed by elimination.
• Cage compare (3.8) and the rule of parity (4.0) toward the top of the band.

You are ready for hard when medium grids rarely send you back to the chart for basic sums.

Tips for this level

• Write the remaining total of each box beside the board once its cages are placed.
• A cage with two cells left is often the quickest lead.
• Update notes after each placement instead of in batches, so a stale candidate cannot mislead you.

Next step

Ready for grids with no givens? Expert Killer is next. For the same band without sums, try hard classic Sudoku.