> [!WARNING]
> **Warning:** You're viewing the deprecated Manifest V2 version of this article. See [Manifest V3 - Manifest key](https://developer.chrome.com/docs/extensions/reference/manifest/key.md.txt) for the MV3 equivalent. The Chrome Web Store no longer accepts Manifest V2 extensions. Follow the [Manifest V3 Migration guide](https://developer.chrome.com/docs/extensions/develop/migrate.md.txt) to convert your extension to Manifest V3.

This value can be used to control the unique ID of an extension, app, or theme when it is loaded
during development.
**Note:** You don't usually need to use this value. Instead, write your code so that the key value doesn't matter by using [relative paths](https://developer.chrome.com/extensions/overview#relative-urls) and [extension.getURL](https://developer.chrome.com/docs/extensions/mv2/reference/extension.md.txt#method-getURL).

To get a suitable key value, first install your extension from a `.crx` file (you may need to
[upload your extension](https://chrome.google.com/webstore/developer/dashboard) or [package it manually](https://developer.chrome.com/docs/extensions/packaging)). Then, in your [user data directory](https://www.chromium.org/user-experience/user-data-directory),
look in the file `Default/Extensions/EXTENSION_ID/EXTENSION_VERSION/manifest.json`. You will see the
key value filled in there.