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Killer sudoku versus classic sudoku

Classic sudoku starts with a scattering of digits and asks you to finish the grid. Killer sudoku starts with cages: dashed groups of cells, each with a sum in the corner, and usually with no digits at all. The row, column, and box rules never change. Each still holds 1 through 9 once. What changes is the information you are given and the first ten moves of a solve.

In classic sudoku the early game is scanning: where can this 4 go, which house is almost full. In killer the early game is arithmetic with a short memory. Which cages have only one combination? Which row is missing a single cell from an otherwise complete set of cages, so that cell must make the row add to 45? You still end by placing singles. You start by creating them.

That is why killer feels harder for the first week and then, for many players, calmer. There is always a small sum to audit. You are rarely staring at an empty house with eight candidates. The cage has already forbidden half of them. The cost is that a wrong digit hides inside a sum that still looks plausible, so checking the last cell of a cage against the box is not optional.

Difficulty does not transfer in a straight line. A person who is fast at classic X-Wings can be slow at innies, and a person who likes mental arithmetic can outpace them on killer within a month. The expert end of killer does borrow classic tools — locked candidates, pairs, and eventually fish — but only after the cage logic has narrowed the grid. Learning those classic names is useful. Learning them first is how people start guessing.

If you are coming from classic sudoku, play easy killer until the unique combinations are boring. Then play medium, where a one-cell 45 or a pointing digit is the hardest step. Hard on this site means the puzzle genuinely needs a two-cell innie, a split cage, or a locked digit inside a non-unique cage, several times. That is a different afternoon from a classic hard, and it should be.

Both games are better when the puzzle is known to be unique and known to be solvable by the techniques it claims. That standard is identical. Only the techniques change.

Play a medium puzzle or open the combination sheet.