Abstract: A method and apparatus for representing two or more related markup language source files in a single delta file is provided. The markup language source files are represented in such a way that the delta file is easily processed to produce one or more merged combinations of the source files. The method and apparatus ensure that any one of the original files can be extracted from the delta file and the delta file remains a valid delta file for any remaining source files. The method and apparatus is adapted to enable the sources files to be aligned with respect to structurally significant elements and textual content so as to reduce repetitions in the delta file of common content. The method and apparatus are suited for use, but not exclusively, with XML documents and files.