@romstar said in What's the correct aspect ratio I should be using as a config for a HAPP / VISION PRO 19" LCD arcade monitor for MAME Horizontal / Vertical games:

I changed the bottom line to mame2003_tate_mode = "enabled" which ultimately resolved my issue. now The vertical game is finally displaying the way it should on my HAPP Vision pro monitor mounted vertical !! At some point I'd like to build a vertical only list of games and have the menu / retropi running exclusively in vertical mode.

I knew it would change your life. I would not recommend trying to make the whole of retropie itself work in vertical mode yet, though. I stayed with my rotating monitor because rotating emulationstation causes performance issues. I believe it also became more difficult to rotate emulationstation on the Pi4 because the option was deprecated. I stayed with my Pi3b for my vertical MAME games for that reason and hope that a good solution for a fully vertical system presents itself later.