Interactive Strong-Link Lesson
When Strong Links Stand Tall or Fly
An X-Wing crosses two perfectly aligned links. When the alignment bends, those same two-place restrictions can become a Skyscraper or a 2-String Kite.
The Shared Foundation
Two Places Still Means One Must Be True
Choose one digit and scan for rows or columns where it has exactly two possible homes. Each pair is a Strong Link: the two ends cannot both be true, but they cannot both be false. One end must contain the digit.
Pattern One
Skyscraper: Parallel Strong Links
The two Strong Links rise in parallel columns:
r2c4–r9c4r3c8–r9c8
The lower ends line up across Row 9. At least one upper end—the two roofs at r2c4 and r3c8—must therefore be 6.
r2c9 and r3c5 each see both roofs, so neither can contain 6.
Picture it: two parallel towers share a foundation line, while their uneven roofs control anything that sees them both.
Pattern Two
2-String Kite: Perpendicular Strong Links
Now one Strong Link crosses a row and the other travels down a column:
- Row link:
r3c5–r3c8 - Column link:
r2c9–r5c9
r3c8 and r2c9 meet inside Box 3. That shared box is the kite. Because the box can contain only one 6, those two inner endpoints cannot both be true.
Follow the strings outward to r3c5 and r5c9. At least one remote endpoint must be 6. Their opposing intersection, r5c5, sees them both and therefore loses the 6.
Picture the flying kite: the shared box is the kite, the perpendicular links are its strings, and the victim waits at the opposing intersection of the tails.
Two for One
A Second Kite Hiding in Plain Sight
A keen eye may notice that the same pattern also works with candidate 3. The four endpoints happen to contain the identical pair (3, 6), so they form one 2-String Kite on 6 and another independent 2-String Kite on 3.
For candidate 3, the Strong Links are again
r3c5–r3c8 across Row 3 and
r2c9–r5c9 down Column 9. Their remote endpoints,
r3c5 and r5c9, are both seen by
r5c5. Therefore, r5c5 also loses 3.
The additional 3 at r5c1 does not disqualify the pattern.
Row 5 is not required to be a Strong Link; it only allows the victim at
r5c5 to see the remote endpoint at r5c9.
Double consequence: neither Kite depends on the other.
The same four cells simply happen to form a valid Kite for both 3 and 6,
eliminating both candidates from r5c5 each independently due to each independent Kite.
Interactive Check
Which Pattern Are You Seeing?
Two column Strong Links remain parallel. One endpoint from each aligns in the same row.
One row Strong Link and one column Strong Link have endpoints inside the same box.
The victim sees both remote endpoints at the opposing intersection of the tails.
The two unaligned endpoints act as roofs and force the elimination.
Before You Eliminate
Check the Entire Connection
- Both links must concern the same digit.
- Each Strong Link must truly offer exactly two homes in its row or column.
- For a Skyscraper, the links stay parallel and one end of each must see the other.
- For a Kite, one link is a row and one is a column; one endpoint from each must share a box.
- The victim must see both remote forcing endpoints.