You
            can set the aspect ratio of the pixels that the Player view or fullscreen
            Player uses, either square and non-square pixels. 
         
         NoteYou can only toggle
               the pixel aspect ratio in the fullscreen Player if you entered it
               through a Player view. 
            
         To set the pixel aspect ratio in a Player view:
         
         
            - In the Player view or fullscreen Player,
               middle-click or press the tilde key (~) to display the Gate UI and
               go through the south gate.
            
- In the Player options, select the Display
               tab.
            
- Under Interaction, select Aspect Ratio
               to use non-square pixels. In this case the Player displays the image
               using the pixel aspect ratio defined for the composition. You can
               examine this ratio in the Pixel Aspect Ratio field in the Composition
               tab of the Tool UI. Deselect Aspect Ratio to use square pixels.