txt From the 70's to 2007 the open source .txt filename extension format commonly referred to plain text documents encoded using the ASCII character set. To support internationalization and localization, .txt text documents are today text files encoded using the UTF-8 or UTF-16LE standard which is a superset of the ASCII character set. Text documents of type .txt typically have minimal formatting for example no support for bold or italic characters or support for bullet points etc. This allows .txt documents to use minimum storage space and be platform independent as long as the operating system supports the underlying encoding character set used to create the .txt document. On windows .txt file support has existed since 1985 when Windows 1.0 was released and since then has been mostly associated with the notepad application on Microsoft Windows.
docx In an effort to create an open document standard, Microsoft in collaboration with ISO/IEC and Ecma, developed the Office Open XML standard in 2006. One of the filename extensions supported in this specification is the .docx extension, a text document filename extension. The .docx was introduced in Microsoft Office Word 2007 and has been supported ever since in later iterations. It has become the default filename extension for all text documents produced using Microsoft Office Word. Given the open source nature of the XML specification, more alternative document processing applications support read and write capabilities on documents saved with the .docx filename extension. This is in comparison to the .doc filename extension which is a proprietary asset owned by Microsoft.