So for the past decade or so Microsoft has been using the .doc/.xls/.ppt/etc format for Office files, with minor changes at most in each version. In Office 2007, when they overhauled everything, they introduced OOXML, with new file extensions (.docx/etc) and a completely different architecture. Despite complaints of inconsistencies, areas that could be improved, and useless legacy features, OOXML is on the fast-track to ISO approval (in which case it would be an international standard for documents) and has been approved in several countries under mysterious circumstances.