Unique Rectangle Type 1
A Unique Rectangle Type 1 is the simplest uniqueness elimination: four cells at the corners of a rectangle share the same two candidates, and one corner carries extra candidates. That rescue cell cannot hold either deadly-pair digit, so both are removed from it.
What is Unique Rectangle Type 1?
A valid Sudoku has exactly one solution. That guarantee is a constraint in its own right: any arrangement of candidates that would allow two interchangeable solutions cannot be part of the answer. The most common shape that threatens uniqueness is the Unique Rectangle — four cells at the corners of a rectangle, each containing the same two candidates, with the corners split across two rows, two columns, and (critically) two blocks so that swapping the pair inside the rectangle produces a second valid grid. This interchangeable arrangement is called a deadly pattern.
Type 1 is the simplest case. Three of the four corners hold only the deadly pair, while the fourth — the rescue cell — has one or more extra candidates beyond the pair. Because the deadly pattern cannot be part of the solution, the rescue cell cannot contain either of the deadly-pair digits. If it did, the rectangle would collapse into the deadly pattern. So both deadly-pair candidates are removed from the rescue cell, often leaving it as a naked single.
How to spot it
Scan for rectangles: pick two rows and two columns whose four intersection cells all contain the same two pencil-mark candidates. The rectangle must span two different blocks in both directions — if all four cells sit in the same block, the block's own constraints prevent the deadly pattern, so the technique does not apply. Once you find a matching rectangle, check whether exactly one corner has extra candidates. If so, you have a Type 1: delete the deadly pair from that cell. Rectangles with two or more rescue cells lead to Types 2, 3, or 4 instead.
The logic
If the rescue cell held either digit of the deadly pair, the remaining three corners would be forced to fill in the pair in the only way that satisfies their rows and columns — and that fill-in is the deadly pattern, which has two interchangeable forms. A puzzle with a unique solution cannot contain a deadly pattern, so the rescue cell must avoid both digits of the pair. Removing them from the rescue cell is the only safe play, and it follows directly from the uniqueness guarantee without needing to know which configuration is correct.