Also known as “fighting capability” and “combat ability”, fighting ability in Dungeons & Dragons is used in the general sense to denote the combat abilities of a fighter and also in the specific sense as a measure of the probability of a character scoring a hit in combat. That is to say, a character with a fighting ability of six is sometimes said to fight as though a sixth level fighter, but this usually does not encompass the hit die size, saving throws, and rate of attack that the fighter class enjoys. For instance, the Boot Hill and Gamma World conversions in the first edition Dungeon Master’s Guide distinguish fighting ability from saving throws and hit dice, but seemingly not from attack rate. On the other hand, the “experience” entry in the glossary uses fighting ability in the more specific sense of an increase in effectiveness on the attack matrices, and that is often the usual sense intended. Interestingly, this statistic has not remained very consistent across editions, neither with regard to classes nor monsters. The cause can probably be traced to its occurrence as a bridge between Chain Mail and Dungeons & Dragons, where fighting ability is expressed in multiplications of “men” and in terms like “hero” and “wizard”.
In the Chain Mail Fantasy Supplement it is stated that a hero has the fighting ability of four figures and a superhero that of eight figures, which is fairly straightforward, the value of a figure depending on armament; all other fantasy unit classifications, including wizards, have specific ratings. For Dungeons & Dragons, ten levels of fighting ability were allocated to the fighter, corresponding to ten experience levels. At first level a fighter was rated as “man +1”, at second level “2 men +1”, at third level “3 men or hero −1”, and so on. The exact significance of the “+1” at levels one and two remain elusive, but magicians, clerics, and thieves all start out with the rating “man” before advancing to “man +1” at level two. By way of analogue, monsters simply attack as their hit dice, with any plus or minus added to one roll, as explained on page six of Monsters & Treasure, meaning that a goblin (HD 1−1) attacks once and deducts one from the roll, whilst an ogre (HD 4+1) attacks four times and adds one to a single roll. This only applies when they are fighting “normal men” or the equivalent, however, and the fantasy combat table is used whenever more powerful types face off against one another.
The alternative combat system takes a different approach, with fighters advancing in ability every three levels, clerics and thieves every four levels, and magicians every five levels; normal men are treated as first level fighters, and this first bracket is equivalent to THAC0 19. Similarly, orcs (HD 1) and goblins are rated at the same level of fighting ability. With Swords & Spells this was changed with normal men, orcs and goblins being rolled back to THAC0 20, though first level magicians, clerics and thieves remained equivalent to the fighter. This also resulted in monster THAC0 being capped at ten on reaching fourteen hit dice, as opposed to nine in the original game, and reflected a one-hundred percent success rate against armour class nine. It is unlikely to be a coincidence that this was also the THAC0 of fighters of levels thirteen to fifteen, though they were themselves capped at nineteenth level with a THAC0 of six. The classic versions of Dungeons & Dragons (B/X, BECMI, and the simulacrum Labyrinth Lord) adopt the change made to normal men, but class everything up to one hit die as THAC0 19 and do not distinguish between fighters and the other classes with regards to starting fighting ability.
With the first edition of Advanced Dungeons & Dragons, though, there was a rethinking of the situation, so that first level thieves, magicians, and normal men were classified as equivalent to monsters of lower than goblin fighting ability, which is to say THAC0 21; fighters and clerics were rolled back to THAC0 20, equivalent to a goblin, whilst orcs and other single hit die creatures were classified as having the original THAC0 19 once common to all. Even with their relatively speedy level advancement, it appears that Gygax calculated the fighting ability of thieves so that it would never overtake that of clerics with the same experience point total, a significant demotion. The cleric, thief and magician could advance until they had a THAC0 of nine, ten, and eleven, respectively, but the fighter could continue to advance in fighting ability up to level seventeen, when he achieved a THAC0 of four. Despite a strong start, monsters were capped at sixteen hit dice with a THAC0 of seven, though for every “plus three” after their hit dice they moved up a bracket. The intent seems to have been to make fighters stronger at higher levels, but also make all classes somewhat weaker at lower levels relative to monsters.
That the result was not very satisfactory to Gygax seems fairly obvious from the changes he instituted in Unearthed Arcana, particularly the introduction of weapon specialisation. With second edition all the classes were brought into line with goblins at first level, as well as men-at-arms and all other monsters of less than one hit die. The fighting ability of thieves was increased in advancement granularity at a ratio of 1:2 to levels, with the result that they became erratically related to clerics, sometimes ahead, sometimes equal and sometimes behind. Similarly, the fighting ability of magicians was increased in advancement rate at a ratio 1:3 to levels, but with less noticeable effect. The advancement rate of monsters was also made cleaner by hit die, though for some reason did not take the obvious step of 1:1 granularity, which OSRIC eventually adopted. With the virtual integration of weapon specialisation into the second edition fighter class his previous ability to attack as many creatures of less than one hit die (one hit die or less in the original game) as he had levels was eclipsed and that last vestige of the multiple men approach taken by Chain Mail was removed from the default rules and became truly optional.
It has often seemed to me that the system can be improved upon without going much beyond any of the paradigms already explored. That a fighter ought to start with a better fighting ability than the other classes seems desirable and the Chain Mail approach suggests as the equal of a 1+1 hit die monster. Considering the frequent complaints about the combat viability of low level thieves, it would also be a small matter to return them to their previous standing relative to clerics. So, if all classes start off at level one with a fighting ability of one (FA 1) and if this is equivalent also to a single hit die monster, then the rest almost writes itself. HD 1−1 indicates FA 0, equivalent to level zero, and HD 1+1 indicates FA 2, with fighters similarly having the equivalent +1 to hit mirroring the effects of weapon specialisation in that regard. In fact, HD 1+1 could also denote FA 1(2) with +1 damage and the standard statistic shorthand of the game would be largely unaffected. Advancement for each class would be 1:1, 1:2, and 1:3 with respect to fighters, clerics as well as thieves, and lastly magicians. An overview of the changes between editions and comparison with the potential aforementioned alternative can be downloaded here.