Hard

Skyscraper

What it is

Two columns (or rows) where a digit has only two places, with one end of each in the same row – the other two ends remove the digit from cells that see both.

How to spot it

Find a digit with exactly two candidates in each of two columns. If two of the cells share a row (the base), one of the other two cells (the tops) must be the digit.

Example

Basis
Removed / placed

In columns 1 and 4, 6 has only two places each, and r7c1 and r7c4 share row 7. So r2c1 or r3c4 is 6, and 6 is removed from cells that see both: r2c5 and r3c3.

Used from level: Hard