A variety of software tools commonly used in research and industry allow a user to select (usually contiguous) segments of content to be annotated, referenced, or otherwise distinguished from a containing document. However, digital libraries (DLs) often curate only full documents, not these selected sub-documents. Thus, sub-documents in a DL may not have the full complement of metadata, and they may not be visible using DL browse and search facilities. We are interested in explicit representation of sub-documents in a DL environment. In this paper, we show how sub-documents may be represented and curated. We focus on the explicit representation of what we call a mark - an encapsulated address of a sub-document along with associated context. Our contributions are: a software architecture for representing marks as first-class objects together with regular documents in a DL; and an implementation of our architecture using existing software packages with modest enhancements. This approach provides new capabilities for the DL with minimal modification to tools and interfaces familiar to the DL user.