An X-Chain is a chain that stays on one digit, alternating strong and weak links — a special AIC.
You already know the shortest forms: Skyscraper, Two-String Kite, and Crane (strong–weak–strong).
Before this: Skyscraper, Two-String Kite, and Crane are already short X-Chains.
Why it works
Strong links: exactly one end is true. Weak links: not both true. Alternating them from one end to the other means the two endpoints cannot both be false.
If the chain is strong at both ends, any cell that sees both endpoints sees a true candidate either way — so it cannot be that digit.
How to spot it
Start from bi-locals (rare strong links), connect with weak links (see-each-other), keep strong–weak alternation.
Demonstration
Follow strong and weak links for a single digit into a useful chain. Prefer starting at bi-locals.
The green cells are an X-Chain on 6 — same AIC idea as a long Skyscraper family chain.
The alternating links force a truth at the ends: both ends cannot be false. Shared neighbors get stung either way.
Eliminate 6 from cells that see both ends (red).
Example 1
Build an alternating chain on one digit (strong–weak–strong…).
This X-Chain on 3 gives an elimination.
Remove 3 from cells that see both ends (red). At least one end must be true.
Example 2
Build an alternating chain on one digit (strong–weak–strong…).
This X-Chain on 1 gives an elimination.
Remove 1 from cells that see both ends (red). At least one end must be true.