Uniqueness techniques

A valid puzzle has one solution. If four cells in two rows, two columns and two boxes held only the same two digits, you could swap them to get a second solution. So puzzles cannot contain that deadly pattern.

Unique Rectangle types look for three corners with the two digits plus extra candidates and use the ban to remove one of the two digits from the fourth corner (type 1), or to restrict extra candidates in two corners (types 2 to 6). These steps are only valid when the puzzle is known to have a unique solution.

Tip: Hodoku Revisited marks uniqueness-based hints so you can tell they rely on that assumption.

Notation

Uniqueness Test 1: 9 can be removed from r3c5.