What is the difference between Killer Sudoku and regular Sudoku?

Q: What is the difference between Killer Sudoku and regular Sudoku?

Killer Sudoku uses the same 9x9 grid and no-repeat rules as classic Sudoku but replaces all given digits with cages - Groups of cells with a target sum. Digits cannot repeat within a cage. No cells start pre-filled.

Killer Sudoku and classic Sudoku share the same grid and basic rules - But the way information is provided is completely different. Classic Sudoku gives you digits directly. Killer Sudoku gives you sums.

Rules Both Share

9×9 grid
Digits 1-9 in each row, column, and 3×3 box exactly once
No digit may repeat in any row, column, or box

What Killer Adds

No given digits - Every cell starts empty
Cages - Groups of 2-9 cells each marked with a target sum
Cage no-repeat rule - Digits cannot repeat within a cage even when cage cells span different units
Cage arithmetic - The digits in a cage must sum exactly to its label

Techniques That Transfer

All standard Sudoku techniques work in Killer once candidates are established: naked and hidden singles, naked pairs, X-Wing, and everything else. The Killer-specific techniques are applied first to establish candidates, and then standard techniques take over.

Techniques That Are New

The 45 rule and its applications (innies, outies, multi-region sums) are unique to Killer Sudoku. Cage combination analysis - Working out which digit sets produce a given sum with unique digits - Has no parallel in classic Sudoku. Both new techniques are learnable in a single session.