Sudoku X — also called Diagonal Sudoku — plays exactly like classic Sudoku, with one extra rule. The same 9×9 grid, the same digits 1 through 9, the same row, column, and 3×3 box uniqueness; plus the two diagonals (top-left to bottom-right, and top-right to bottom-left) each carry the same uniqueness constraint, so every digit 1 through 9 appears once on each diagonal.

The diagonal cells are marked with a dotted coral ring on the board, so you can see at a glance which cells participate in the extra constraint. There are seventeen cells in total on the X — nine on each diagonal, with the centre cell shared.

In practice the diagonal rule helps more than it hurts. Once you've placed a few digits on a diagonal, the remaining cells on that diagonal often shrink to a small candidate set. Look for the diagonal when row, column, and box scanning stalls — it's often the unit that breaks a stuck puzzle open.

Hard puzzles rely on intermediate techniques — locked candidates, naked pairs, X-wings. Expect to pencil-mark systematically rather than keep candidates in your head.

If a puzzle feels stuck, step away for a minute. The eye catches patterns on a second pass that a long stare never surfaces.