Queens: regions that own a row
If a region fits inside one row, that row’s queen belongs to the region. Cross out the rest of the row.
How it works
Every region needs a queen. If all of a region’s cells are in one row, the region’s queen is in that row.
Every row has exactly one queen. So the row’s queen is the region’s queen, and no other cell of the row can have one. Cross them all out.
The same works for columns. And it works in reverse: if a row’s open cells all lie inside one region, the region’s queen is in that row, and the region’s other cells are out.
The reverse direction
Row 5 below lies entirely inside region E. Row 5 needs a queen, so E’s queen is in row 5. E has only one queen, so E’s cells outside row 5 are all out.
Two regions, two rows
The idea scales. If two regions fit inside the same two rows, those rows’ two queens belong to them. Cross out every other cell in both rows.
Worth memorizing
A region inside one row owns that row’s queen.
Cross out the rest of the row.
Same for columns.
Reverse it: a row inside one region clears the rest of the region.
Practice
0 of 4 rightRegion E lies in one row. Which cells can you cross out?
Region C lies in one column. How many cells can you cross out?
Every cell of column 1 is in region A. How many other cells of A can you cross out?
Where does region A’s queen go?