What does it mean when a colour is trapped in one row or column?

If every open tile of a colour lies in the same row (or the same column), that colour's queen must end up in that line. A line holds only one queen, so it is now reserved for that colour, and you can cross out every other tile in the line. You don't need to know which tile the queen will take.

An example

queenalready ruled outplay herecross out nowtiles to look at

Every open tile of the jade colour is in row 1, so its queen must go in row 1. No other colour can use row 1: cross out the rest of it.

Why it works

The colour needs one queen. All its possible tiles are in one line, so its queen is in that line. The line can't take a second queen, so no other colour's tile in that line can be used.

How to spot it

• Look at thin colours first: a strip of two or three tiles in a row is trapped from the start.
• Re-check a colour whenever it loses tiles. A big colour often becomes trapped once a nearby queen removes part of it.
• Check columns as well as rows.

Related techniques

The mirror image is a line trapped in one colour. The same idea with two or three colours at once is pairs and triples.