[I wrote this up a few years ago for an old friend. We played 1e together for many years, and he was trying to get his head around 5e.] A few observations that may help orient: The main resolution mechanic is an “Ability Check,” where the DM sets a target number (“DC”, Difficulty Class) and the player attempts to roll equal or more on 1d20 +/- modifiers. Unlike the good old days, this same mechanic is used for just about everything, from attacks to skill checks (pick pocket, climb wall, open door) to saving throws, etc. So, for example, instead of THAC0 the target number is the opponent’s AC. High AC good, low AC bad. Advantage/Disadvantage means roll two d20s and take the higher/lower roll, which translates to roughly +4/-4 bonus, but more exciting. “Proficiency Bonus” is just a compact way to represent level vs ability tables. In the good old days, for example, fighter THAC0 decreased every three levels, cleric every four, magic-users every six. The single Proficiency Bonus tabl...