Apple Aperture 2 - Manual (Page 97)

Chapter 3 Working with Aperture Projects 97
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You can also create albums within projects to help you organize your images into
relevant groups. For example, images in your Soccer project can be divided into three
albums: Game 1, Game 2, and Game 3.
You can also place versions from other projects into an album that resides within a project.
What Are Folders?
In Aperture, you use folders to organize projects and albums. For example, you can
import images into projects and then place the projects in folders based on photo type
or location. If you shoot multiple projects for the same client, you can create a folder
that holds the client’s individual projects.
Folders contain only albums, projects, and other folders. They don’t contain masters
or versions.
Three albums created
within one project
These folders organize
images based on type
and location.