>>5883344Roll under vs Roll over is a very interesting dichotomy. Similarly to THAC0 and Ascending AC, on the surface they appear identical, and mathematically they are, but are actually different in play and in the mental space of the game.
For example, Roll over and adding modifiers is more popular then Roll under and adding a negative number or adding a number to the target because adding is just a bit easier then subtraction in your head. If you're told to roll a die and add 2 for your skill bonus and 1 for your stat and you get a 7+2+1 you can instantly figure it us 10, as opposed to an identical Roll under of 13-2-1. It's faster.
Secondly, positive modifiers also have a psychologically uplifting or boon effect, and can be scaled to infinity within the same number set. ie- your high level character with a 1d6 sword can add +22 because of all their badass skills and level ups just fine. You can do the same thing for Roll under, but if you consider that additions to the target number you'll run into a point where you cannot fail (d20 Roll under 22). While ascending can do the same thing (d20 Roll +19) you still get degrees at a glance of how "good" you Roll, where as Roll under a roll of one will always be the best you can do regardless of anything you can add or subtract from the Roll.
In my opinion, Roll under (especially percentile) is a perfect fit for more realistic or "tactical" games, as percents have a more modernist feel to them, and character progression is essentially shaving off your imperfections and dialing in on consistently where as Roll over feels more fantastical and heroic.