Two-Strong-Link Chains
A two-strong-link chain is an AIC (alternating inference chain) involving a single candidate digit and two strong links, both of which are conjugate pairs. In an AIC, strong and weak links alternate, and the chain must begin and end with a strong link. An AIC therefore contains at least four nodes (propositions) and three links in the pattern strong-weak-strong. The name refers to the two strong links in this pattern. Two-strong-link techniques involve only a single candidate digit and can be regarded as the shortest X-Chains.
If you draw the strong links on the board, they often form some recognizable shapes:
- Skyscraper: the two strong links are parallel.
- Two-String Kite: the two strong links are perpendicular.
- Turbot Fish: one of the two strong links is tucked inside a box.
Another two-strong-link chain, the Empty Rectangle, uses grouped nodes for one of its strong links.