Skyscraper
A Skyscraper is the parallel-house form of the two-node X-Chain. For one digit, two row nodes or two column nodes form (A=B)-(C=D): the equals signs are strong inferences and the dash is a weak inference across the perpendicular house. The chain proves A OR D, allowing eliminations from candidates that see both endpoints.
What is a Skyscraper?
A Skyscraper uses two same-digit bilocation nodes in parallel rows, or in parallel columns. Write it as (A=B)-(C=D). Each base house has exactly two positions for the digit, giving the two strong links (XOR) A=B and C=D. The connected candidates B and C share a perpendicular house and cannot both be true, giving the weak inference (NAND) B-C. This is also the AIC representation of two Siamese Sashimi X-Wings.
How to spot it
For one digit, find two rows with exactly two candidate positions each, or use the transposed column form. Verify that one side of each strong-inference node shares the same perpendicular house; that pair supplies the weak inference. The other sides are endpoints A and D. Remove the digit only from a target that sees both endpoints.
The logic
If A is false, B is true because A=B is a strong link. Since B-C is a weak inference, C is false; C=D then makes D true. Therefore A OR D is true, possibly both. Any same-digit candidate seeing both endpoints can be removed. This is the Type 1 same-digit elimination. Swapping rows and columns leaves the proof unchanged.