What is an .md file?

MD stands for Markdown. An .md file is a plain text file that uses ordinary punctuation to mark up formatting: a # for a heading, an asterisk for a bullet. Nothing else is inside it.

What is actually in the file

Open an .md file in Notepad or TextEdit and you see what the author typed, punctuation and all. There is no hidden formatting layer, no compression and no proprietary structure, which is precisely the point. A Markdown file is readable whether or not you have the right software.

That is the difference from a Word document. A .docx is a zip archive of XML that needs an application to interpret it. An .md file is just characters.

How to open one

Any text editor will open it, because it is text. What most people actually want is to see it formatted (headings rendered as headings, lists as lists) and for that you need something that renders Markdown.

• In a browser, on any device: drop the file onto the Markdown editor, or paste its contents into the Markdown preview to read it rendered. Nothing is uploaded; the file is read locally.
• Windows: Notepad shows the source; VS Code and Obsidian render it.
• macOS: TextEdit shows the source; MacDown and iA Writer render it.
• Phones: the browser route works everywhere, which is usually the fastest option.

Windows and macOS both associate .md with a plain text editor by default, so double-clicking shows the raw punctuation rather than an error. That is not the file being broken.

Make .md files open here by default

The last paragraph is the annoying part: double-click a Markdown file and you get raw punctuation in Notepad or TextEdit. You can change what opens it, and one of the options is this page.

All Markdown installs as an app. It is the same site, the same code and the same promise, in a window of its own that works offline, and installing it registers it with the operating system as something that can open .md, .markdown, .txt and their relatives. Nothing is uploaded when a file opens that way either: the operating system hands the file to the page, and the page reads it where it already is.

1. Open the Markdown editor in Chrome or Edge on a desktop and use the Install button in its toolbar. Chrome also offers it from the icon at the right-hand end of the address bar.
2. Windows: right-click any .md file, Open with, Choose another app, pick All Markdown and tick Always use this app.
3. macOS: select a .md file, press Cmd+I, set Open with to All Markdown and press Change All.
4. ChromeOS: installed apps appear in the Files app's Open with list straight away.

Safari and Firefox do not implement this yet, so the install step is only offered where it can actually work. Everywhere else, dragging the file onto the editor does the same job in one gesture and needs nothing installed.

.md, .markdown, .mdown

All the same format. .md is shortest and by far the most common. Tools treat every one of these extensions identically, so renaming between them changes nothing.

Where you will run into them

README files in software projects are Markdown, which is why nearly every repository has a README.md. Static site generators, note apps like Obsidian, documentation systems and chat tools all use it. So do large language models: when ChatGPT or Claude replies with headings and bullet points, it is writing Markdown, which is why pasting that reply somewhere often leaves stray asterisks behind.

Converting it to something else

Open the file in the Markdown editor and use Export. PDF, Word and standalone HTML all convert in your browser, so the document never leaves your device, worth knowing if it is a contract or an unpublished draft.