Stacks offer a method of displaying folders or collections of multiple items in the Dock of the Mac. When clicked, the “Stack” will pop open and show the contents sprawling out of the Dock.
Dock Stacks always display on the right-side of the OS X Dock and they default to contain things like the Applications folder, Downloads folder, but many users add a Documents folder or even a Recent Items Dock menu stack.
The default for most stack items is set to Automatic, which means it will change as the folder (or stack) item changes to accommodate the contents. But you can also just choose to set the style of the Dock Stack yourself with a simple right-click on the stack item you wish to change in OS X.
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