There are several ways to look and compare your images in Aperture's Main Viewer.

By default, it is set to Show Multiple. Whenever you select one or several images from the Browser, it will be shown in the Viewer. You can view from 1 to 11 images at any one time. If you select more than 11 images, a notification will appear that let's you know how more images are selected but not shown in the Viewer.

You can change your Viewer mode to Show One. At any one time, despite several selected images, this will show only your primary select.

Another Viewer mode is Three Up. When you select an image from the Browser, it will automatically show you the images before and after it. However, if you select the first image, it will only show the image next to it in the sequence; and when you select the last image, you will only be shown the image before it.

Then there's the Compare mode. Your primary select remains in the viewer while you browse through other images one at a time. This allows you to compare your select with the other images.

And finally, there's the Stack mode, where you can compare your Current Pick (the first image in a stack) with individual images within that same stack.

All these viewing options are available in your Main Viewer, and each is easily accessible through an easy-to-remember combination keyboard shortcut of Option key plus each of the letter that spells U-R-H-O-T. These are all the second letters of the name of the viewing mode based on previous Aperture versions.

If you have a dual-monitor setup, the Viewer Mode pop-up menu will also provide options for the second monitor. These options, in relation to the first monitor, are: Mirror, Alternate, Span, Blank, and Desktop.