**tl;dr** `brew tap` allows you to add more GitHub repos to the list of formulae that `brew` tracks, updates and installs from. When naming tap repos and formulas, however, there are a few gotchas to beware of.
* If you run `brew tap` with no arguments, it will list the currently
tapped repositories. Example:
```bash
$ brew tap
homebrew/dupes
telemachus/desc
telemachus/vim
```
* If you run `brew tap` with a single argument, `brew` will attempt to
parse the argument into a valid 'username/repo' combination. If the argument is a valid name, then `brew tap` will attempt to clone the repository and symlink all its formulae. (See below for what it means to be a 'valid name'.) After that, `brew` will be able to work on those formulae as if there were in Homebrew's canonical repository. You can install and uninstall them with `brew [un]install`, and the formulae are automatically updated when you run `brew update`. (See below for a few exceptions on names and installation.)
* In you run `brew tap --repair`, then `brew` will check for dead symlinks
and relink all valid formulae across all your taps.
* You can remove a tapped repository using the `brew untap` command.
## Naming conventions and limitations
`brew tap username/repo` employs some shortcuts and has some limitations.
* When you use `brew tap` on the command line, you can leave out the
'homebrew-' prefix in commands.
That is, `brew tap username/foobar` can be used as a shortcut for the long version: `brew tap username/homebrew-foobar`. The command will automatically add back the 'homebrew-' prefix.
## Formula duplicate names
If your tap contains a formula that is also present in master, that's fine, but it means that you must install it explicitly.
For example, you can create a tap for an alternative `vim` formula, but in that case when you install from there you must run the command with a more explicit installation target:
```bash
brew install vim # installs from Homebrew/homebrew
brew install username/repo/vim # installs from your custom repo
