BUG+2

Also called BUG +2.

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

The same idea when two squares break the pattern instead of one. You cannot say which is right on its own, but you can rule things out from the pair.

The exact rule, for stronger players

The rule, stated exactly: Two cells carry a third candidate. At least one of the extras must be true, which licenses eliminations from cells seeing both when the extras share a digit.

What it removes: When both extras are the same digit, remove it from cells seeing both. Otherwise the constraint is weaker and must be combined with other logic.

Why it works: If both extras were false the grid would be a BUG. 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

BUG+1Say every open square had two numbers left, and each number had two homes in every group.

Related uniqueness

BUG+3The same again with three odd squares.
Deadly PatternA shape that would give the puzzle two answers if it ever appeared.
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.

How often does it come up?

Our rough estimate: rare.

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