Skyscraper
The Skyscraper follows the same logic as the XWing. It looks similar  like an XWing in which one of the four sides is crooked. It is regarded as a difficult higher method.
Like many other methods, the Skyscraper also has two possible perspectives: seen from the row or from the column. From the row perspective the picture is a horizontal skyscraper.
Criteria for line orientation:
We are looking for two lines that contain the same candidates exactly twice. So we're talking about four occurrences of a candidate in two lines. Here two of the occureances are in the same column, the other two in different columns (otherwise it would be an XWing). The two occurences that are in the same column (green in the examples), are the foundations of two Skyscrapers. The other two occurences (those that are in the other columns, pink in the examples) are the roofs of the Skyscrapers.
Just like the XWing, this pattern can only be solved crosswise. Since the foundations are in the same column, only one of them can, in the end, be filled in with the candidate. Thus, in the absence of other options, that candidate must go in the roof field in the other Skyscrapers. In other words, the candidate will be in either the first foundation and the second roof, or in the second foundation and the first roof.
If you now entered this candidate in any other field that affects both roof fields (that is to say, the canididate was eliminated in both roof fields by this field), then this situation would mean that both foundations have to be filled in with this candidate. This is a contradiction, because both foundations are in the same column.
So the candidate can be eliminated from all fields which share a logical unit with the two roof fields.
It is probably not necessary to reformulate the criteria on column orientation. The pictures will explain it better.
In the following image there are four possible starting points for Skyscrapers with the 6 as candidate.
Here from the row perspective. We're looking for two rows that contain the candidate exactly twice.
The foundations are green, the roofs are pink, and the yellow fields are affected by eliminations, because they share a logical unit with two pink fields.
If the 6 were entered into one of the yellow fields, it would have to be eliminated from both pink fields. Then the green fields would both have to be filled in with the 6.
This is the contradiction.


Also from the row perspective. green = foundation
pink = roof
yellow = candidate for elimination


Here from the column perspective. We're looking for two columns that contain the candidate exactly twice.
green = foundation
pink = roof
yellow = candidate for elimination


Once again, from the column perspective.
green = foundation
pink = roof
yellow = candidate for elimination
It almost sounds a bit implausible that the 6 can actually be eliminated from all the yellow fields, right?
But the logic is indisputable. It works.


This is what the result looks like.
In this scenario, the Skyscraper was really a great help. It was the breakthrough. The rest is easy.



