Tier 1
Container completion
Rows and columns need queens too. One uncrossed square left in a line means the queen goes there.
Region completion applied to the other two containers. A row holds exactly one queen; a column holds exactly one queen. So a row with a single uncrossed square is resolved, whatever colours are involved.
Three kinds of container, same rule:
| Container | Count | Resolves when |
|---|---|---|
| Row | one queen | one square left |
| Column | one queen | one square left |
| Colour region | one queen | one square left |
Why players miss it
Colour is loud and rows are quiet. The regions are painted; the rows and columns are not. So people scan the colours obsessively and forget that a row four squares from the bottom has been reduced to one option three moves ago.
A cheap habit fixes it: after each placement, run your finger down the edge of the board and count the surviving squares in each row, then across the top for each column. On an 8×8 that takes about four seconds and it catches most of what you would otherwise miss.
Counting is the whole game
Nearly every advanced technique on this site is counting in a fancier costume. Simple undercounting counts colours against rows; the rule of crowding counts rows against colours. Get comfortable counting containers now and the tier-3 material stops looking exotic.