Taking numbers away
Hitori is a Japanese puzzle that runs the opposite way to most number grids. You start with every square filled and remove numbers by shading them until no row or column shows a number twice. Two limits keep you honest: shaded squares can never share a side, and the unshaded squares must stay joined in one piece.
On this site the board is a 6×6 grid using the numbers 1 to 6. Because every square already holds a number, each choice touches its row, its column and its neighbours at once, so deductions chain quickly.
Patterns that settle squares instantly
A handful of shapes appear on almost every board.
- The sandwich. When a row reads something like 4 1 4, the middle square always stays unshaded. One of the two 4s has to go, and whichever it is, it sits next to the 1, so the 1 can never be shaded too.
- The adjacent pair. Two equal numbers side by side, such as 3 3, cannot both be shaded, because shaded squares never touch. So one stays. That means any other 3 in the same row must be shaded.
- The triple. A run of three equal numbers in a line forces the ends to be shaded and the middle to stay. It is the sandwich and the pair rule working together.
After each shading, circle the squares touching it. A circle is only your note that a square stays, but it is useful: a circled number is now the one copy that survives in its row and column, so every other copy of that number in those lines must be shaded.
A worked chain on the 6×6 board
Say the top row reads 2 5 2 6 1 3. The sandwich tells you the 5 stays, so circle it. Now suppose the second column, below that 5, also contains another 5 further down. Since the top 5 is staying, that lower 5 must be shaded. Shade it, then circle the squares touching it. One of those circled squares might be a 4 that also appears elsewhere in its row, which gives you the next shading. This is how most Hitori boards go: one forced square sets off a short chain, and you follow it until it stops.
Keeping the white squares joined
The connection rule is the one beginners forget, and it is often what finishes a board.
- Corners are fragile. A corner square touches only two others. If one of them is shaded, the other must stay unshaded, or the corner would be cut off.
- Edges need care too. A square on the edge has three neighbours. If two are shaded, the third must stay.
- Diagonal lines of shading are a warning. Shaded squares may touch at corners, but a diagonal chain of them can slice the board in two. Before shading a square that extends such a chain, check that the unshaded squares on either side still connect.
When two shadings look equally possible, test each against connection. Often one would wall off a square.
Mistakes and how to recover
The board tells you a lot. A number drawn in clay with a tick under it still repeats somewhere in its line. Red means a broken rule: touching shaded squares, a shading that splits the unshaded area, or two circled copies of the same number in one line.
- Shading before checking neighbours. Before you shade, glance at the four squares touching it. If one is already shaded, stop.
- Circling on instinct. Circle only squares you have proved must stay. A wrong circle hides the real answer.
- Clearing every clay number first. It is tempting to work through repeats one by one. Instead, look for sandwiches and pairs first; they are certain, while a lone repeat usually is not.
Practising well
Since the board is always 6×6, improvement comes from speed of recognition, not from bigger grids. Try a few boards where you only use the sandwich, pair and circle rules, and see how far they carry you. Then add the connection checks.
Each puzzle has one solution. A hint settles one square and pins it with a green corner, and Undo can take it back. When you use one, ask which pattern you missed. If you like the joined-region rule, Nurikabe at /games/nurikabe builds a whole puzzle around keeping one area connected.
