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Sudoku technique · Specialty reference · reference
Other rectangle variants
Unique-rectangle types 3, 5, and 6, hidden rectangles, and avoidable rectangles use additional subset, link, or solved-value conditions.
How to spot it
Use the source to distinguish each exact rule.
The common mistake
Their geometry and uniqueness assumptions are not interchangeable. These variants are reference-only here.
Learn this first
Unique rectangle types 1, 2, and 4.
Further reading: hodoku.sourceforge.net