Hacking 2e: Roll-Under Mechanics – Part 1

The inconsistency of die mechanics in AD&D is often cited as one of the more confusing parts of the game. Putting aside the cacophony of die types (death to the percentile roll), the main issue is roll-under vs roll-over and where the modifier goes. This can seem arbitrary on first glance, but there is some consistency. Here are (most of) the rolls one can make, the ‘goal’ of that roll, and what’s being modified:

Modifier for…	Attempting to…	High or Low	Modifying
Saving Throw	Beat a Static Value	High	Roll
Surprise	Beat a Static Value	High	Roll
Attack Roll	Beat a Static Value	High	Roll
Damage	Get a High Number	High	Roll
Reaction	Place on a Spectrum	High (if you fix it)	Roll
Initiative	Get a Low Number	Low	Roll
Nonweapon Prof.	Beat a Static Value	Low	Ability Score
Thieving Skill	Beat a Static Value	Low	Thieving Skill Score
Morale	Beat a Static Value	Low	Morale Rating

Note: The assumption in this breakdown is that we’re gunning for the ‘desirable’ result for the PC.

You can see that we’re mostly trying to beat a value, whether that number is based on rolled attributes, level-based tables, more-than-three, or whatever number THAC0 spits out (depending on how one uses THAC0). Damage and Initiative are just seeking straight-up results from the dice. Reaction is our outlier, as it’s used to determine the temperature of the NPCs.

There’s correlation here: you add the modifier to the roll if rolling high, and you add the modifier to a static value if rolling low. Initiative is the weirdo, being the only time you modify the roll while also trying to roll low.

As I’ve made clear in previous discussions, I’m in favor of making everything into a roll-under mechanic. This chart shows that in order to do that, I’ll need to add modifiers to the values rather than the rolls. We’ve already got three rolls with that setup (NWP, Thieving Skills, and Morale), so let’s do the rest.

To Hit Descending AC

In the last entry, I discussed why I like descending AC and some of its advantages. I left the disadvantages for later, and so we have arrived.

My argument is simple: the bad parts of descending AC are the complicated ways used to determine how one beats the numbers, and ascending AC is therefore identified as the superior method only because it resolves the annoying math. However, I do not believe that one necessarily follows from the other; you can have descending AC without the complications. Before we get to that, let’s run through some of these methods and see where they cause friction, starting with the two methods that were used during AD&D’s run: To-Hit tables and THAC0.