Unique Rectangle Type 4

Unique Rectangle Type 4 occurs when one of the deadly pair digits forms a conjugate pair in the roof cells' shared house. Since that digit is locked in the roof, the other deadly pair digit can be eliminated from both roof cells.

Technique

1. Find a potential deadly rectangle with candidates {X,Y}.
2. Two floor cells have only {X,Y}; two roof cells have extras.
3. Check if digit X (or Y) forms a conjugate pair in the roof cells' shared row, column, or block.
4. Since X must be in one of the roof cells, eliminate Y from both roof cells.

Tips

• Type 4 relies on one deadly pair digit being a conjugate pair in the roof's house.
• The locked digit prevents the deadly pattern, so the other digit is safe to eliminate.
• This often produces more eliminations than Type 2.