Interlinear text has long been considered a valuable format in the presentation of multilingual data, and a variety of software tools have facilitated the creation and processing of such texts by researchers. Despite the diversity of tools, a common core of editorial functionality is provided. Identifying these core functions has important implications for software engineers who seek to efficiently build tools that support interlinear text editing. While few applications are specifically designed for the creation or manipulation of interlinear text, a number of tools offer varying degrees of incidental support for this modality. In this paper we provide a comprehensive set of critieria upon which the derivation of functional criteria can be based. We describe the basis on which a group of tools was selected for investigation, along with the evaluation criteria. Finally we consolidate our findings into a functional specification for the development of software applications for the editing of interlinear text.