Skyscraper

Level 6 · Orange Belt (Green Stripe)

Strategy:

Single-Digit Patterns

Requires:

X-Wing

Learn after:

Two-String Kite

Overview

Skyscraper is a single-candidate pattern using two conjugate pairs. When two rows each have exactly two cells with a candidate, sharing one column but not the other, eliminations are possible from cells seeing both "roof" cells.

How It Works

  1. Find two rows where a candidate appears in exactly two cells each.
  2. Two of the four cells must share the same column (the "base").
  3. The other two cells (the "roof") must be in different columns.
  4. Eliminate the candidate from cells that see both roof cells.

Example

Starting position

Look for Skyscraper.

1

Scan columns 6 and 8 for 9

Check digit 9 in columns 6 and 8.

2

Two conjugate pairs on 9

Each column has exactly two cells with candidate 9, forming conjugate pairs: one end of each pair must be true.

3

The pairs meet in row 8

The pairs share row 8 as a common endpoint, forming a Skyscraper pattern.

4

r5c6 or r6c8 must be 9

Either r5c6 or r6c8 must contain 9, so these cells that see both cannot keep 9: r6c4.

5

Remove 9 from cells seeing both ends

Remove 9 from the cells that see both r5c6 and r6c8: r6c4.

6

Result

Result after applying the technique.

Tips & Patterns

  • Skyscrapers are easier to spot than X-Wings when columns don't align.
  • The pattern resembles two buildings with different heights.
  • One of the two roof cells must contain the candidate.