How to play Diagonal Sudoku

Play Diagonal Sudoku

The rules

The grid is a 6×6 square split into six boxes of 2 rows × 3 columns. Some cells already hold digits: these are the givens. Fill every empty cell so that each digit from 1 to 6 appears exactly once in:

• every row;
• every column;
• every 2×3 box;
• both diagonals, the corner-to-corner lines. Their cells are shaded and crossed by a thin line.

Every puzzle has exactly one solution, and logic alone is enough to reach it. You never have to guess.

How to play here

• Select a cell: click it, or use the arrow keys.
• Enter a digit: keys 1–6, or the number pad below the grid.
• Notes (small candidates): press N or the Notes button. In notes mode, a digit you type is recorded as a small candidate. Shift + digit also writes a note.
• Erase: Delete, Backspace or 0.
• Undo and redo: Ctrl+Z and Ctrl+Y (on a Mac: ⌘Z and ⌘⇧Z).
• Hint: each press of Hint tells you more: first where to look, then which technique works there, and finally the digit itself. If there is a mistake on the board, the hint shows you where.

A digit that repeats in a row, column, box or diagonal is marked with stripes and a wavy underline.

Difficulty levels

The level is set by the hardest technique the puzzle requires, not by the number of givens:

Easy puzzles have 14 givens, so they are easy to start.

Techniques

Full house

A row, column, box or diagonal has one empty cell left. It takes the missing digit.

Hidden single

Within a unit, a digit fits in only one cell. Look at the boxes first, then at the rows, columns and the two diagonals. The diagonals are easy to forget.

Naked single

A cell has only one possible digit, because every other digit already appears in one of its units.

Overlap

If, inside a box, a digit can only go in cells that are also on one row, column or diagonal, it must be there. Remove it from the rest of that line. It works the other way round too.

Naked and hidden pairs

Two cells of a unit with the same two candidates take those two digits: remove them from the rest of the unit. Two digits that fit only the same two cells reserve them: remove every other candidate from those cells.

X-Wing

If a digit can appear in two rows only in the same two columns, remove it from every other cell of those two columns.

XY-Wing

A pivot {X,Y} sees wings {X,Z} and {Y,Z}. One wing is always Z, so a cell that sees both wings cannot be Z. A diagonal counts as “seeing”.

Tips

1. Check the diagonals early. A diagonal often has a hidden single that rows and columns do not show.
2. The two cells where a diagonal crosses a box are a good place for an overlap.
3. When you place a digit on a diagonal, check that diagonal too, not only its row, column and box.