What each format actually is
.txt is plain text: characters and line breaks, nothing else. No font, no bold, no colour, because the format has no mechanism to describe any of that. It is the smallest of the three by a wide margin and the only one every text editor on every operating system, including ones decades old, can open without a second thought.
.rtf, Rich Text Format, adds formatting on top of plain text using
human-readable control words rather than hidden binary. A bold word is wrapped in something
like {\b bold} right inside the file. Fonts, colours, alignment and lists all
work this way, which is why .rtf survives being opened in almost anything built after the
early 1990s, when the format was introduced specifically to move formatted text between
otherwise incompatible word processors.
.docx is Microsoft Word's format since Word 2007: a zip archive holding several XML files, one describing the document body, others handling styles, images and metadata. It supports considerably more than .rtf, tables, embedded objects, tracked changes, comments, but that richness comes from a more complex structure that a simple text-first editor is not built to fully reproduce.
What survives, and what gets dropped
Convert a heavily formatted .docx down to .rtf and most of the visible formatting, bold, italic, colour, alignment, lists, comes through intact. What gets dropped is anything specific to Word's richer feature set: tracked changes, comments, and embedded objects like a chart or an OLE-linked spreadsheet. Convert further down to .txt and every piece of formatting is gone, deliberately, since the format has nowhere to put it.
That loss is not always a downside. A resume or a form field expecting formatted input can reject or mangle rich formatting it never asked for, where plain text pastes cleanly every time. Knowing which direction you are converting, and why, matters more than assuming richer is always better.
The one that survives the longest, unopened
If a file needs to sit untouched for a long time and still open correctly whenever it is next needed, .txt and .rtf have the advantage: both are plain, documented, decades-old formats with no dependency on a specific company's software staying available. .docx is also an open, published standard (Office Open XML), so it is not fragile in the way an old proprietary binary format would be, but it assumes something Word 2007 or later, or software that reads that schema, still exists to open it. For a note or a document you genuinely never want to lose, .rtf is usually the better bet of the two formatted options: it keeps the formatting .txt cannot hold, without the archive-and-XML complexity .docx depends on.
Choosing one for a specific job
- A quick note or a config file: .txt. Nothing else is justified.
- A document with bold, headings or a bullet list, meant to be read by anyone: .rtf. It opens in Word, LibreOffice, TextEdit, Google Docs and a browser editor alike.
- A document you will keep editing collaboratively, with tracked changes or comments: .docx, since that is the format built to hold them.
Working with all three in one place
The WordPad replacement on this site opens and saves .rtf and .docx directly, and reads plain .txt the same way any text editor does, all in a browser tab with nothing installed and nothing uploaded to a server. For notes that never need formatting at all, the plain online notepad saves as .txt with no extra step.