CodeIgniter: Multiple Applications, how to set $application_folder?

I'm planning to follow Method 2 as described here: https://github.com/EllisLab/CodeIgniter/wiki/Multiple-Applications, which will result in my folder structure being:

/application/
    /frontend/
    /backend/
/system/
开发者_运维技巧

The wiki further states that,

"Both frontend.php & backend.php are duplicate of the index.php, where you’ll need to update the $application_folder in each file pointed to the corresponding folder"

My question: in the given example, what exactly should the values of $application_folder be?

If you don't changed anything big in the index.php file, the default is by line 43.

The default is "application",

I think that in your case it's "frontend" & "backend"