Empty Rectangle: inspect the missing candidate positions

Track one digit in the box

An Empty Rectangle uses box candidates confined to an intersecting row and column, together with an outside strong link. The intersection itself may contain the candidate. Check that the tracked digit has no box positions outside those two lines.

HoDoKu — Single Digit Patterns · SudokuWiki — Empty Rectangles

Our middle-box example for 6

Let the middle box allow 6 only at D4, D6 and F6, with no placed 6. Row H allows 6 only at H2/H6. Consider D2.

• If H2=6, column 2 blocks D2.
• If H2≠6, H6=6 removes D6/F6. The box then forces D4=6, blocking D2 through row D.

Both branches remove 6 at D2. These are partial candidate restrictions, not a full puzzle.

An extra position opens an escape

If E5 also permits 6, H6=6 need not force D4: the middle box could choose E5 instead. D2 is no longer blocked by the same argument. Do not overlook the central cell just to retain an L-shaped outline.

Explain it before naming it

Check the entire box, then follow both ends of the outside pair. If both branches block one target, you can explain the deduction without treating the technique’s name as a picture to memorize.

In Classic 9×9, use Scan to scan the current entries, inspect justified eliminations and open this lesson. Chain and ALS searches are bounded; UR Type 1 and BUG+1 require verified uniqueness. Reading this lesson never changes your saved game.

Try before looking

References and further reading

Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.

• HoDoKu — Single Digit Patterns
• SudokuWiki — Empty Rectangles