Unique Rectangle Type 2

Walkthrough

Example 01 — Two corners, one extra

This rectangle uses numbers 2 and 6, its two non-diagonal corners in column 9 have an extra number 9. If neither corner were 9, all four corners would reduce to numbers 2 and 6, producing the deadly rectangle.

Therefore one of those two corners must be 9. Any other cell that sees both corners cannot also be 9. In this case, number 9 can be eliminated from cells r5c8 and r6c8, which are in the shared visibility zone.

What to notice: The elimination is outside the rectangle. The rectangle establishes that at least one of the two corners must take the extra candidate.

Once the two corners are identified, stop looking at the rectangle itself. Trace the cells that share a house with both exceptional corners.

Those cells form the elimination zone. Remove the extra candidate only there; cells that see one corner but not the other are not covered by the proof.

Walkthrough

Example 02 — Find every cell that sees both

This rectangle uses numbers 2 and 4, its two non-diagonal corners in column 7 have an extra number 1. To prevent a deadly rectangle pattern, one of them must take number 1. Therefore, number 1 can be removed from cell r3c7 because it sees both corners

What to notice: The target does not need to see all four rectangle corners. It only needs to see both cells that carry the shared extra candidate.