Empty rectangle
A digit's candidates inside a box collapse onto one row and one column.
An empty rectangle is a configuration inside a single box: the candidate cells for one digit all lie on a single row and a single column within that box, tracing an L (or a plus). The four "corners" of the box untouched by that row or column have no candidate for the digit — those are the "empty" cells.
On its own, the configuration says: within this box, the digit lands either somewhere on this row or somewhere on this column. Pair the pattern with a strong link elsewhere — a row or column outside the box where the digit has exactly two candidates, one of which lines up with the L — and the move pinpoints a single cell that can be eliminated.
When the move applies
Empty rectangles turn up regularly in advanced puzzles. Scan each box for an L shape; then look outside the box for a strong link that the L can lever against.
The procedure
- Find a box where a digit's candidates trace a single row + single column inside the box.
- Look for a strong link in some row or column outside the box that crosses the L.
- The intersection of the strong link's far end with the L's axis identifies the elimination.