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4e-inspired THAC0/AC system

by Giampaolo Agosta

The THAC0/combat table and AC system is, with the Saving Throws and skill system, one of the chief bones of contention, as it is often considered awkward.

I've taken into consideration the 4e system (a single progression with semi-static bonuses for Fighters), and have tried to generate a similar system for OD&D (i.e., one that produces more or less the same hit probabilities and the same difference between fighters and other classes).

The system is not playtested, but should be sound.

Character base attack progression
All characters, regardless of class, have a base attack bonus equal to 1/2 their level, rounded up

Tier-based combat training bonuses
At each tier, characters focusing on combat training get the following bonuses (non-cumulative!):
Basic: Fighters, Dwarves, Elves +1; Magic Users -1
Expert: Fighters +2
Companion: Fighters +3
Master: Fighters +5

Attack bonus formula
Attack bonus = 1/2 level, round up + combat training bonus + ability score bonus

Monster attack progression
Monsters receive an attack bonus equal to their HD.

Bonus damage
In OD&D, high level characters do bonus damage when fighting enemies they could automatically hit, if it wasn't for the "1 always misses" rule. The same is easily obtained here, with a rule similar to power attack of newer editions:

If AB > AC : AB-AC damage bonus

Obviously, AC is reversed to simplify the computations. AC grows from 11 up, and is computed as 20 - Old AC

Armor AC
No armor 11
Padded 12
Leather 13
Chain 15
Plate 17
Full 20
Full + Tower Sh. 22
Full +5, T. Sh. +5, Ring of protection +5 37

Thus, the maximum AC is around 37 (40 with high dexterity), while the maximum attack bonus is 31 (18 + 5 fighter + 3 strength + 5 magic weapon).

Here are some quick numbers to show typical fighters (with appropriate magical weapons) targeting armored enemies (with magical armor and rings of protection at higher levels).

Basic (2nd level): +2 vs AC 16 -> to hit 14 -> hit rate 35%
Expert (9th level): +9 vs AC 22 -> to hit 13 -> hit rate 40%
Companion (20th level): +16 vs AC 25 -> to hit 12 -> hit rate 45%
Master (31th level): +22 vs AC 33 -> to hit 11 -> hit rate 50%

The above assumes no magical equipment at basic levels, +1/+2 equipment at expert level, +1/+3 equipment at companion level and +3/+5 equipment at master level.

It's easy to see that hit rate improves slightly with tiers, which is IMO a good thing, as it offsets the increased amount of HP.
At 36th level, a Fighter should have a base bonus of 24 (2/3 his level), if one used the OD&D progression. Here, he arrives at 23. A cleric or thief gets exactly the same bonus as in the standard table (basically, the cleric progression is extended to all characters), except their progression is at a a finer grain (+1 every 2 levels instead of +2 every 4).
A Magic User gets an improved hit probability (+17 instead of +15), but not by much.

Demihumans need to progress up to 36th level, and are similar to Fighters at basic/expert levels, and to Clerics at higher levels, as in standard OD&D. Halflings are more difficult to mirror, since they progress as Fighters at lower level and as Magic users at higher levels in standard OD&D, and are therefore changed to progress as Clerics (this should be also more balanced).

The main difference between this progression and the original OD&D one is in the nonlinearity that you have when low level characters attempt to hit very low AC, which is different. However, I don't think it is worth reproducing, since low-level characters would be killed in short order by anything that had AC -5 or better...