HOW DO YOU CREATE A STACK?

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A stack is basically a folder which you have put on the Dock to have fast access to it. To get a folder as a stack in the Dock, open Finder and navigate to the folder which contains the folder you want to make a stack. Then, drag the folder to the right part of the Dock, so a stack will show up

Once a folder is in the Dock, you may need to control-click on its icon and select "Display as Stack".