You can specify whether the Viewer displays one photo, three photos, or multiple photos at a time. If your system uses multiple displays, you can configure your main and secondary displays to show single photos, three photos, or multiple photos simultaneously. The display settings you choose also control the display of photos in full-screen view.
To display multiple photos: Choose View > Main Viewer > Show Multiple (or press Option-U).
This is the default setting. When Show Multiple is chosen, the Viewer can display anywhere from one photo up to twelve at a time, depending on the number of photos selected in the Browser.
To display a single photo: Choose View > Main Viewer > Show One (or press Option-R).
When the Viewer is set to Show One, the Viewer displays one photo at a time regardless of the number of photos selected in the Browser. This setting allows you to make a multiple photo selection in the Browser and then view each photo by itself within the selection.
To display three photos: Choose View > Main Viewer > Three Up (or press Option-H).
When the Viewer is set to show three photos at a time, Aperture displays the selected photo in the center of the three photos that appear in the Viewer.