Skyscraper

Skyscraper is the first in a trio of Sudoku solving techniques that consist of two strong links, and one weak link.

If you're not familiar with Strong and Weak links, you may find it useful to read about those first.

The '9' candidate appears twice in column 2, this means that if one of these '9' candidate is false, the other '9' candidate must be true, i.e. this is a strong link.

The '9' candidate appears twice in column 9, another strong link.

The '9' candidate appears more than twice in row 5, this means that if one of these '9' candidate is true, the other '9' candidates must be false, i.e. this is a weak link.

If we assume the '9' in cell A is false,

⇒ The 9 in cell B is true.

⇒ The 9 in cell C is false.

⇒ The 9 in cell D is true.

Separately, if we assume the '9' in cell D is false,

⇒ The 9 in cell C is true.

⇒ The 9 in cell B is false.

⇒ The 9 in cell A is true.

This means that the value in cell A or cell D (or both) must be '9'.

If the value of cell A is '9', we would remove all the other '9' candidates in these cells.

If the value of cell D is '9', we would remove all the other '9' candidates in these cells.

If the value of cell A or D (or both) must be '9', this means we can remove all '9' candidates where these two areas overlap.

This is another example of a Skyscraper. We know that cell A or cell D (or both) must have the value '8'.

This allows us to remove all '8' candidates in the cells that 'see' both cells A and D.

Skyscrapers can also appear upside-down.

And, they can appear sideways.

It's also worth mentioning that since all Strong links are automatically Weak links, we can also apply Skyscraper with three Strong links.

(You could argue that this is an example of Simple Colouring)

Skyscraper is the first in a trio of techniques that consist of Strong-Weak-String Links (the other two being Two-String Kite and Crane).

Skyscrapers can appear in four different orientations, but you are looking for,

Two horizontal or vertical Strong links, and they need to be in the same orientation (one horizontal and one vertical Strong link won't work).
A horizontal or vertical Weak link that links your two Strong links

Can you spot the Skyscraper in this Sudoku?

Did you spot it?

Can you spot the Skyscraper in this Sudoku?

Did you spot it?

Practice puzzles

Here are 10 Sudoku puzzles where you will need to apply the Skyscraper technique to solve,

Sudoku Academy

Skyscraper

Skyscraper

Practice puzzles