We examine methods of implementing queries about relational databases in the case that these queries are expressed in first-order logic as a collection of horn clauses. because queries nay be defined recursively straightforward methods of query evaluation do not always work and a variety of strategies have been proposed to handle subsets of recursive queries we shall express the essential property of capture rules is that they can be applied independently thus providing a clean interface for query-evaluation systems that use several different strategies in different situations we show how rules suggested previously can be fit into this framework and we propose some new capture rules and generalizations of old ones.