One of the means of reducing the information retrieval time is by taking advantage of the property of inclusiveness of information retrieval systems. When one knows the system response to a query which is more general in relation to another query, then in an inclusive retrieval system in order to retrieve the response to the more specific query it suffices to limit the information retrieval process to the search of the system response to the more general query. This paper demonstrates the property of inclusiveness of document retrieval systems where documents are indexed by unweighted descriptors, and in which query search patterns are Boolean functions of descriptors (systems using the Inverted File Method, the Canonical Structure File Method, or the Sequential File Method). The paper presents three methods for determining a partial ordering relation on a set of Boolean search patterns of queries, implying a partial ordering on the set of the system responses to these queries and discusses the adequacy of each method depending on the information retrieval method used. The paper also proves the general theorems concerning the property of inclusiveness with regard to the class of information retrieval systems considered. Furthermore, the methods for calculating the degree of generality/specificity between queries are given. The utilization of the property of inclusiveness may considerably reduce the operating costs of information retrieval systems (particularly systems using the Sequential File Method, e.g. the SDI systems). Moreover, the results of the studies presented in the paper give the possibility of gradual narrowing or broadening in an automatic way of a given Boolean search pattern of a query, which is of vital importance for on-line information retrieval systems.