Unique Rectangle
Four cells in a 2-row × 2-column × 2-box rectangle sharing the same two candidates form a deadly pattern. Eliminate anything that would lock in that pattern.
Advanced sudoku technique
What it is
A Unique Rectangle (UR) is a pattern that would create TWO valid sudoku solutions if it appeared in the final grid — and since every legitimate puzzle has exactly one solution, the pattern can't occur. The setup: four cells forming a rectangle spanning two rows × two columns × two 3×3 boxes (so two cells in each box), where all four cells share the same two candidates {X, Y}. If those four cells ended up as {X, Y, Y, X} or {Y, X, X, Y}, both arrangements would solve the puzzle equally well — a 'deadly pattern' with two solutions. So the pattern must be broken: at least one of the four cells must hold a candidate OTHER than X or Y. That third candidate forces a unique placement and lets you eliminate X and Y from the relevant cells. UR has several sub-types (Type 1 is the most common: three cells have only {X, Y} and one cell has {X, Y, Z}).
When to use it
On expert and master puzzles. UR sub-types differ; Type 1 is the standard first attempt.
Worked example
Cells R2C3 = {4, 7}, R2C8 = {4, 7}, R5C3 = {4, 7}, R5C8 = {4, 7, 9}. The four cells form a UR rectangle (rows 2 and 5, columns 3 and 8, boxes 1 and 6). If R5C8 ended up as 4 or 7, the deadly {4,7} pattern would have two solutions. Since the puzzle is uniquely solvable, R5C8 must hold the extra candidate — R5C8 = 9.
Try it
The Sudoku247Online solver walks you through every move of any puzzle one logical step at a time, naming the technique that justifies each placement. Paste a puzzle to see this technique applied in real time.