by Cliff McQuesten
Related Values Chaining extends xy-chaining to analyze row-, column- and box- interactions between chain links and cells that are not part of the chain. A group of two or more chains are considered. Related values analysis can solve puzzle cells that are not links in the chain, and can determine additional cells that can be used as chain links leading to longer chains.
Tree Chaining extends xy-chaining for multiple branches and incorporates Related Values Chaining. A “Chain Tree” is a multi-branched chain.
These strategies are described at SudokuChaining.YahooSites.com.