A Crane is a short chain on one digit: strong–weak–strong. Skyscrapers, Two-String Kites, and Cranes are basically the same family — different drawings of the shortest useful X-Chain / AIC.
Before this: Same turbot family as Skyscraper and Two-String Kite.
Why it works
Strong links mean “exactly one is true.” Weak links mean “they cannot both be true.” Alternating them forces a useful conclusion at the ends: at least one end is true, so cells that see both ends cannot hold the digit.
If you understand Skyscraper, you already understand Crane — same logic, different geometry.
Demonstration
For one digit, follow a strong link, then a weak link, then another strong link.
The green cells form a Crane on 5 — a tiny turbot / X-Chain.
The chain says at least one end must be true for 5. Shared neighbors of both ends are unsafe either way.
Eliminate 5 from cells that see both ends (red).
Example 1
Find a short strong–weak–strong chain on one digit.
This Crane involves 3.
Remove 3 from cells that see both ends (red). One end must be true.
Example 2
Find a short strong–weak–strong chain on one digit.
This Crane involves 1.
Remove 1 from cells that see both ends (red). One end must be true.