Apple Aperture 2 - Manual (Page 29)

Chapter 1 An Overview of Aperture 29
I
Comparing Photos
Aperture allows you to carefully compare images. You select an image in Compare
mode to have Aperture display it for comparison against other images in a selection.
While comparing images, you can rate the selected images or make adjustments to
them. For example, you might use the comparison method to make final selections
of the best images in a series. Or, you might compare two versions of an image and
make adjustments.