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Grouping applications in Plank

Plank, elementary's dock, has the ability to group applications. These grouped applications look like this:
When you click on it, you'll see all the applications in the folder: 
To do this, open the folder /usr/share/applications in "Files". Here, you will see most of your applications. Select all the applications you want to group and copy them.
After they've been copied, navigate to your Home directory and create a new folder. The name of the folder should be the same name as the Plank group you are creating. For example, if you're doing a group for multimedia, you should name the folder "Multimedia".

Open the newly created folder, and paste the files you've just copied. Finally, go back to your Home directory and Drag-and-Drop the folder into Plank. That's it!