Skyscrapers have this name because they resemble two buildings of unequal height.
(although they may be on their side or upside down).
The two 'buildings' in this example are in columns. Each 'building' (column) must have a strongly
linked candidate (6 in this case). Strongly linked meaning the candidate occurs exactly
twice in each of the two columns:
The base of both 'buildings' must be in the same row (row 5 in this example).. The tops of the
buildings must be in different rows (otherwise we would have an X-Wing instead of a
Once you have identified two 'buidlings', you can ignore the rest of the columns. There could be
other columns with strongly linked 6's but these don't affect our skyscraper, even if they share the
same base (row 5).
Note that the 6's do not have to be strongly linked in row 5 (they are weakly linked
in this example since there are four 6's in row 5).
This forces at least one of the 'building tops' (blue cells) to be a 6. It is possible that both blue cells are 6's because
the 6's in row 5 are weakly linked. (If the 6's in row 5 were strongly linked, then only one of the
blue cells could be a 6).
Any cell that is visible to both blue cells can have 6's elimnated.