What is a .md file?

A .md file is a plain text file that contains Markdown. You can open it in any text editor, and the .md extension simply tells apps to treat the text inside as Markdown. It is not a special binary format.

It's just text with formatting characters in it

Open a .md file in any editor and you see exactly what's stored: words, plus a few characters that mark up structure. A # starts a heading. A - starts a list item. Two asterisks make text bold. That's the whole idea. Markdown is a way to write formatting using normal keyboard characters, so the source stays readable on its own.

Here is what the raw text of a small .md file looks like:

# Shopping list

Milk and eggs, plus:

- Bread
- **Coffee** (the good kind)
- Apples

That's a heading, a short paragraph, and a bulleted list with one bold item. The file holds exactly the characters above. An app that understands Markdown reads them and shows you the formatted version.

The extension is the signal, not the format

The .md on the end of the filename is the file extension. It doesn't change what's inside the file. The bytes are the same plain text either way. What the extension does is tell your Mac, and the apps on it, "open this as Markdown."

Rename notes.txt to notes.md and the contents don't change at all. You've only changed the label. The reverse is true too: a Markdown file saved as .txt is still Markdown text, it just isn't flagged as such.

You may also see .markdown, which means the same thing:

README.md
notes.markdown

Both are Markdown files. .md is far more common because it's shorter.

Why "plain text" matters

Plain text means the file isn't locked to one program. A .docx needs Word, or something that can read Word's format. A .md file opens in anything that reads text: a code editor, a plain note app, the Mac's own tools. Nothing is hidden inside, nothing is encoded.

Portability is the whole point. Your writing stays yours, in a file you can still open in twenty years with whatever editor exists then.

In Unmarked

Unmarked opens a .md file and renders it as a formatted document, with the raw source one shortcut away (⌘E to toggle). It reads .markdown, .mdown, .mkd, and plain .txt as Markdown too, so a file saved under any of those extensions behaves the same. Point Unmarked at it and you get the text, formatting and all.