Skyscraper
The Skyscraper works with one digit and two parallel lines. Take two rows in which the digit appears as a candidate in exactly two cells each. If one cell from each row shares a column — the aligned bases — while the other two cells, the roofs, stand in different columns, you have a skyscraper: two towers of unequal height joined at the ground.
The logic is a short chain. Suppose 3 appears exactly twice in row 2, at r2c1 and r2c5, and exactly twice in row 7, at r7c1 and r7c6. The bases r2c1 and r7c1 share column 1; the roofs are r2c5 and r7c6. The bases cannot both be 3 — they'd clash in column 1. So at least one row must place its 3 on the roof instead: if r2c5 is not 3, then r2c1 is, forcing r7c1 off and r7c6 on. One roof or the other is always 3.
That disjunction powers the eliminations: any cell that sees both roofs can never hold the digit. Here r1c6 and r3c6 see r7c6 by column and r2c5 through the top-middle box, while r8c5 and r9c5 see r2c5 by column and r7c6 through their box — all of them lose 3. The pattern also exists with two columns and row-aligned bases, eliminating symmetrically.
Skyscrapers are quick to scan for once you track conjugate pairs — lines holding a digit exactly twice. They are in fact the shortest X-Chain, so mastering them is the natural gateway to chain techniques.
- Hunt lines where the digit has exactly two candidates; two such parallel lines with one aligned end form the pattern.
- The roofs must be in different columns (or rows) — perfectly aligned ends form an X-Wing instead, with different eliminations.
- Eliminations hit only cells seeing both roofs — usually found via box overlap near either roof.