A square whose star would leave some row, column or region with no room for its stars is empty. A star crosses out everything around it, and on one-star grids its whole row, column and region too; sometimes that’s everything a small region had left.
This is one of the Star Battle techniques. Each page shows its technique on the same two-star 8×8 from the game, Hard level 34.
The rule
- Pick a square near a small region or a nearly full line.
- Imagine a star there, and cross out what it would rule out.
- If some row, column or region would be left without enough room for its stars, the square is empty.
In a real puzzle
On the 8×8, it’s the very first move. Region a needs two stars. A star in row 2, column 2 would touch all six of region a’s other squares, leaving no room for the second. So that square is empty.
How often you’ll need it
On 39 of the 50 Easy levels, 42 Medium and every Hard one: 4.2 times per Easy level on average, 5.3 on Medium and 14.3 on Hard, where it’s the busiest technique of all. It’s 8 of the 44 moves on the 8×8 here.