Two-Node X-Chain
A Two-Node X-Chain — traditionally listed as 2 Strong Links — is the general length-2 same-digit AIC. Its signature is (A=B)-(C=D): two strong-inference nodes joined by one weak inference. The chain proves the inclusive endpoint relation A OR D, so the digit can be removed from any candidate that sees both endpoints.
What is a Two-Node X-Chain?
Write the construction as (A=B)-(C=D). Each equals sign is a strong link (XOR node): at least one side is true, and for an ordinary two-position house node the sides are also mutually exclusive. The dash is a weak inference (NAND), meaning B and C cannot both be true. The node houses may be any combination of rows, columns, and boxes; the logical relations, not a bridge drawing, define the chain.
How to spot it
For one digit, find two justified strong-inference nodes and connect one side of each with a valid weak inference. The unconnected sides A and D are the endpoints. A target is valid only when it conflicts with both endpoints. Skyscraper and 2-String Kite are named house-signature restrictions of this general two-node X-Chain.
The logic
If A is false, the first strong link makes B true. The weak inference makes C false, and the second strong link makes D true. Thus A OR D is true: at least one endpoint contains the digit, possibly both. Any candidate that sees both endpoints would conflict whichever endpoint is true and can therefore be removed. This is the Type 1 same-digit elimination.