How to solve a unique rectangle in Sudoku
A unique rectangle uses the rule that a puzzle has one solution to rule out a pattern of four cells that could be swapped.
How it works
Four cells in two rows, two columns and two boxes, all holding only the same two digits, could swap those digits and give a second solution. A proper puzzle never allows that.
So when most of the rectangle holds just the two digits, the remaining corner (or corners) must have something else, and you can remove the pair from them or use the extra digits to make further eliminations. This guide covers the common variants.
How to spot it
Look for cells with the same two notes at three corners of a rectangle that spans exactly two boxes.
A worked example
This is a step from a real puzzle, a moment where the next move needs a unique rectangle.
F are the floor cells (only the two digits), R the roof cells (extra digits). Struck-through digits are the ones that can be removed.
R8C4, R9C3, R9C4 each hold only 2/8. If the rectangle held only 2 and 8, they could swap and the puzzle would have two solutions. A puzzle has one solution, so R8C3 can't be 2 or 8: remove both.
Where it fits in Rocksled Sudoku
Puzzles that need a unique rectangle are never rated below Expert. Each step of this kind adds 78 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.