The sandwich technique says: a number between two of the same, like 4 2 4, stays unshaded. If the middle were shaded, neither outer number could be (shaded squares can’t touch), so both would stay unshaded, and repeat.
This is the first of the Hitori techniques.
The rule
- Look along every row and down every column for three squares in a row whose outer two numbers match.
- Circle the middle one: it stays unshaded.
In a real puzzle
On the 9×9 every technique page follows, it’s the first move. Row 1 starts 6 7 6, so the 7 stays unshaded.
How often you’ll need it
On every one of the game’s 150 levels, which all have at least one sandwich on the empty grid. On the 9×9 here it’s 6 of the 55 moves, and on a Hard level about 3 to 4 on average. Most come at the start: sweep for them all before anything else.