Talk:THAC0/@comment-173.247.14.71-20171211123755

It isnt that complicated. You have a number to hit an AC of zero. You have bonus and penalty that straight up +-. All you have to do is adjust for their actual ac.

say your to hit is 15. You attack someone with ac 5. Your str adds +2 and your weapon adds +1. You roll a 8. Did you hit? base 15 -5 ac target is 10 to hit goal. 8+3 is 11. hit!

Internally many computer games are much more complex than this; some perform in the neighborhood of 20 calculations per attack. You don't see it, though.

Tabletop, ... before battle you figure out what you need to hit and what you add to your die rolls. If nothing changes (usually spell based) its just roll + bonus or roll - penalty depending on which you have. May have to change values when you change targets, but its dead simple in practice.