Description
Once you've set up your screen you can override it's aspect ratio.
Alternative Command
-aspect0
-aspect1
-aspect2
-aspect3
Example
mame <gamename> -aspect 16:9
Command Format
<gamename> -numscreens 2 -screen2 \\.\Display2 -aspect2 4:3. This option will force screen 2 of your game to use a 4:3 aspect ratio. If you specify auto, MAME will try to sort it out itself based on the original screen resolution. -aspect will apply to all screens whilst -aspect0,1,2,3 only applies to the specific screen.
MAME's Default Setting
auto