DOCX is the file format of Microsoft Word. It arrived with Word 2007 and replaced the older binary .doc format that Word 97 to 2003 used; the x at the end stands for XML. Today it is the most common format for editable documents: letters, CVs, reports, contracts and theses travel as DOCX files, and nearly every word processor can open one.
What is inside a DOCX file
A DOCX file is a ZIP archive. Rename report.docx to report.zip, open it, and you will find a set of folders and XML files, each holding one part of the document:
word/document.xmlholds the text itself, with its paragraphs, tables and formatting.word/styles.xmldefines the styles: Heading 1, Normal, List Paragraph and so on.word/media/contains the images, stored as ordinary picture files.docProps/core.xmlcarries the document's properties: title, author, and when it was created and last saved.[Content_Types].xmland the_relsfolders tell a program how the parts fit together.
Inside document.xml, text is stored in runs: stretches of characters with the same formatting. A single word can end up split over several runs after editing, tracked changes or a change of font halfway through, which is why text copied out of a DOCX sometimes behaves in surprising ways.
A standard, not only a Microsoft format
Microsoft submitted the format, called Office Open XML, for standardisation. It was published as ECMA-376 in 2006 and as the international standard ISO/IEC 29500 in 2008. The standard defines two flavours, Transitional and Strict; Word saves Transitional by default, for compatibility with older documents. Spreadsheets (.xlsx) and presentations (.pptx) use the same packaging.
A few relatives share the structure: .docm is a Word document that contains macros, and .dotx is a Word template.
Opening a DOCX without Word
LibreOffice Writer, Google Docs, Apple Pages, WPS Office and ONLYOFFICE all open and save DOCX files. Simple documents look the same everywhere. Complex ones, with text boxes, unusual fonts, fields or tracked changes, can shift slightly between programs, because each one interprets the standard in its own way and not every font is installed everywhere.
How the spell checker helps
You can open a DOCX file in the Spell Checker directly, with no account and without having Word installed. Its text is checked for spelling, grammar and punctuation mistakes, and for style too if you turn on Picky mode. Apply the fixes you want, then download the corrected document as DOCX again: it keeps its formatting, so styles, tables and pictures come back as they were, with only the corrected words changed. You can also download the text in another format, such as ODT, PDF or plain TXT. A single check covers a whole document of up to 150,000 characters.
It is a useful second opinion even if you do have Word: a different checker notices different things, and it covers more than 30 languages. For a full routine before you send the document, see What Is Proofreading?