Killer Sudoku and Sudoku X

Killer Sudoku

A Killer Sudoku usually starts with no digits at all. Instead, the grid is divided into dashed “cages”, each with a small number in its corner: the sum of the digits inside it. No digit may repeat within a cage.

• A one-cell cage gives its digit away directly.
• Small cages have few options: a two-cell cage of 3 must be 1 and 2, one of 17 must be 8 and 9.
• When all but one cell of a cage is filled, the last digit is the sum minus the rest.
• The 45 rule: every row, column and box adds up to 45. If cages almost cover a box, the gap is 45 minus their sums.

Sudoku X

Sudoku X adds the two main diagonals as extra units: just like a row, each diagonal must contain the digits 1 to 9 exactly once. On the board, the diagonal cells are tinted.

• Diagonal cells have more neighbours, so they lose candidates fast — check them first.
• The centre cell sits on both diagonals, its row, its column and its box at once.
• Hidden singles work on the diagonals too: a digit with only one free spot on a diagonal goes there.

Both variants are available in Sudoshi’s single-player mode, the daily challenge and multiplayer rooms with a free account.