Deadly Pattern

Also called Non-unique pattern, Deadly Rectangle.

New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.

This technique assumes the puzzle has exactly one solution. It reasons about what would create a second solution, so it is only valid on a properly constructed puzzle. Do not use it to check whether a grid is well formed.

What it means

A shape that would give the puzzle two answers if it ever appeared. A good puzzle has one answer. So anything that would make the shape must be wrong. That single idea is behind every trick on this list.

The exact rule, for stronger players

The rule, stated exactly: A set of cells whose candidates can be permuted into two or more valid completions without affecting any cell outside the set. The simplest is four cells at the corners of two rows, two columns, and two boxes holding the same two digits.

What it removes: Nothing directly. Deadly patterns license eliminations by exclusion: any candidate arrangement that would complete one is impossible.

Why it works: If such a set existed with no given constraining it, the two arrangements would both satisfy every constraint, so the puzzle would have at least two solutions. This technique is only valid when the puzzle is known to have exactly one solution. It is not a deduction about Sudoku rules, it is a deduction about the puzzle setter's promise. On a grid with multiple solutions it produces wrong answers.

Worked examples

None yet. This pattern does not turn up in our daily boards, so there is no real example to show. We would rather wait than draw a fake one.

Learn these first

Naked PairTwo squares in the same group both hold only the same two digits.

Related uniqueness

Extended Unique RectangleA bigger version using six squares and three numbers instead of four and two.
Hidden Unique RectangleThe rectangle trick again, but the giveaway is hidden.
Reverse BUGThis one uses the numbers you have already filled in, not the ones still open.
Unique LoopLike a rectangle but longer, a ring of squares all sharing two numbers.
Unique RectangleFour squares in a rectangle across two boxes, all holding the same two numbers.

How often does it come up?

Our rough estimate: the umbrella concept behind every Tier 6 technique. The rectangle case is by far the most common.

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