BUG+3

Also called BUG +3.

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 again with three odd squares. Rarer, harder to see, and the reasoning does not change.

The exact rule, for stronger players

The rule, stated exactly: Three cells carry a third candidate. At least one must be true.

What it removes: Weaker still. Useful mainly when the three extras share a digit and a common peer.

Why it works: As for BUG+2, with three escapes rather than two. 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+2The same idea when two squares break the pattern instead of one.

Related uniqueness

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.
Unique LoopLike a rectangle but longer, a ring of squares all sharing two numbers.

How often does it come up?

Our rough estimate: rare, and rarely decisive.

Play today's puzzle or read how difficulty is graded.