A great many automatic indexing methods have been implemented and evaluated over the last few years, and automatic procedures comparable in effectiveness to conventional manual ones are now easy to generate. Two drawbacks of the available automatic indexing methods are the absence of reliable linguistic inputs during the indexing process, and the lack of formal, analytical proofs concerning the effectiveness of the proposed methods. The precision weighting procedure described in the present study uses relevance criteria to weight the terms occurring in user queries as a function of the balance between relevant and nonrelevant documents in which these terms occur; this approximates a semantic know-how of term importance. Formal mathematical proofs are given under well-defined conditions of the effectiveness of the method.