The one-answer rule of the unique rectangle, found another way: by following where one number of the pair can go, instead of counting each corner's options.
123456789
123456789
5
4
7
8
3
5
6
8
7
8
7
3
1
5
4
3
2
6
9
7
5
7
4
5
5
7
3
9
6
7
346
9
5
2
8
1
36
8
1
5
6
7
3
4
9
2
36
4
7
5
3
The rectangle's cornersLoses a pencil mark
Hidden or plain
In a plain unique rectangle, three corners show only the pair. Here they don't need to; the far corner gives itself away through where the 3s can go.
How to spot one
Find a rectangle across two boxes whose corners share a pair. Then check the lines through its corners for a number of the pair with just two places.
Only for proper puzzles
This relies on the puzzle having exactly one answer. Every board in the game does; a puzzle with several answers would break it.
Practise Hidden rectangle
Boards that need it, with the owl's hints when you're stuck.