Apple Aperture 2 - Manual (Page 159)

Chapter 5 Working with Images in the Browser 159
II
When you’re working with the Browser and Viewer together, you can also reposition
the Browser on the screen. For example, you can rotate the Browser to the side.
You can also swap the position of the Viewer and Browser when you want.
To rotate the position of the Browser:
m Choose View > Browser > Rotate Position (or press Shift-W).
To swap the position of the Browser and the Viewer:
m Choose View > Browser > Swap Position (or press Option-W).
Using the Rotate Position and Swap Position commands, you can arrange the Browser
and Viewer in positions most convenient for you.
Browser rotated