Tier 2
Row and column confinement
When a whole row can only be served by one colour, that colour is used up here — and disappears from the rest of the board.
The mirror of region containment, and easy to overlook because it starts from the quiet half of the board.
Take a row that still needs a queen. List the colours of its surviving squares. If every one of them is the same colour — say all four remaining squares in row 3 are green — then row 3’s queen is green. Green has only one queen. So green is finished elsewhere: every green square outside row 3 can be crossed off, however far away it is.
The eliminations this produces are often on the far side of the board, which is exactly why it is easy to miss and satisfying when it lands.
The two directions, side by side
| Start from | You learn | You eliminate |
|---|---|---|
| A colour confined to one line | that line belongs to the colour | other colours in that line |
| A line served by one colour | that colour is spent in this line | that colour outside the line |
Same fact, two useful readings. Checking both after every placement costs seconds and catches a surprising amount.
When it appears
Most often mid-solve, once adjacency has thinned a row down to three or four squares. Very rarely on move one. If you find yourself checking it on a fresh board, go do adjacency elimination first — there is nothing to find yet.