DOCX is a file extension of the Office Open XML (OOXML) document format. OOXML documents are zipped, XML-based file formats developed by Microsoft for representing a variety of office suite types of documents.

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What does X denotes in ASPX, DOCX, XLSX, PPTX etc?

Previously there were ASP, DOC, XLS, PPT etc and now ASPX, DOCX, XLSX, PPTX respectively. What does X denotes in ASPX, DOCX, XLSX, PPTX etc?