Much has been written about the costs and benefits of using pre-coordinated headings when indexing documents. However, to date there have been few attempts at specifying a formal semantics for such systems, and little study of how well the intended meanings of these headings correspond to the meanings understood by both end users and professional library staff. In this paper I discuss some possible interpretations of pre-coordinated headings in the Library of Congress Subject Headings. I review previous studies of end user understanding of subdivided subject headings. These studies raise questions as to how well the intended meanings of these headings are understood by professional librarians and end users. I note methodological issues that may limit the validity of the results and suggest follow-up studies to address these limitations. Finally, I suggest extensions to the W3C ontology for Simple Knowledge Organization Systems (SKOS) to support modeling subdivided headings. I also suggest how concepts modeled using these extensions can be related to classes in a corresponding ontology.