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Strong and weak links: what follows if this is false?
Before reading a web of lines, ask a smaller question: if this candidate is true—or false—what must follow?
The names describe implications, not difficulty
A weak link means two candidates cannot both be true: one true makes the other false. A strong link means they cannot both be false: one false makes the other true. This lesson uses a basic case: a missing digit with exactly two positions in one unit. That pair has both relationships, since exactly one position must hold the digit.
HoDoKu — Chains: links and inferences · SudokuWiki — Weak and Strong Links
Follow 4 through row D
Here is our own partial setup. Row D has no placed 4, and its only possible positions for 4 are D2 and D8. If D2 is not 4, D8 must be 4. If D2 is 4, D8 cannot be 4. Without knowing either condition, we cannot choose D8’s value. Other digits in those cells do not change the stated two-position constraint.
The same pair supports two different steps. Say the condition out loud before following the line; that helps avoid using an implication backwards.
A third position changes the result
Now let D5 also allow 4. Rejecting 4 at D2 leaves both D5 and D8 available, so it no longer forces D8. Setting D2 to 4 still rules it out at D8: row uniqueness has not changed. Check actual entries and complete candidates, rather than assuming your handwritten notes list every position.
Try before revealing
Only D2, D5 and D8 allow 4 in row D. After removing 4 at D2, can you enter 4 at D8?
Show explanation
Not yet. D5=4 is still a counterexample. You need another reason to eliminate D5. If D2 were actually 4 instead, both other positions would be excluded.
Turn a pair into a short argument
Start with a conditional statement and justify each forced step. Stop if a step only feels plausible. Reaching another cell is not itself an elimination; the completed argument needs a usable conclusion. The next two lessons connect two pairs and prove that at least one end of their chain contains the tracked digit.
What Sudoku-Y supports today
The Classic 9×9 Techniques button now scans Skyscraper and Two-String Kite from your current entries, highlights the evidence and links to these lessons. Strong/Weak Links explain the connections used in those proofs; they are not a standalone elimination. Reading or revealing an answer here never changes your saved game.
References and further reading
Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.