The "Man-to-Man" combat system is the portion that Dave Arneson used for his early Blackmoor campaign. The core of the system is expressed in the "Man-to-Man Melee Table" and another table, titled "Individual Fires With Missiles".
The melee table cross-references 12 different weapon types against 10 armor types (two of which are purely for horses). This gives a number, which the attacker must roll equal to or greater than on two six-sided dice in order to kill the defender. The lowest number is 5, for a mounted man with a lance against any armor less than mail, or for a two-handed sword wielder against an opponent in mail; the highest is 12, which shows up only for defenders in plate armor, or for a horse with barding.
The missile table is similar - however, here we see an armor class number, ranging from 1 to 8. What these armor class numbers correspond to is not explicitly given, but the safest presumption is that they are meant to be the same armors shown on the melee table, so class 1 would be "no armor", while class 8 would be "plate armor and shield". The horse armor columns are explicitly labeled, rather than being given as classes.
Each cross-referenced entry on the missile table consists of three numbers, for short, medium, and long ranges of the weapon in question. (These are simply divided in thirds, so, for example, the short bow with a 15" range has a short range up to 5", medium up to 10", and long to 15".) We also see an addition of a "cannot kill" result in this table.
Mounted troops gain an advantage in melee, getting +1 to their roll, while foot troops fighting mounted troops are at -1. Attacks from the rear also grant a +1.
The benefit of first blow (that is, "initiative" in the RPG sense) is gained by the one who initiated combat (by moving into melee range), unless the opponent has a weapon which is "two classes higher", or the defender is fighting from above (e.g., atop a castle wall, rampart, up stairs, etc). The weapon classes are set on the basis of length, so that a defender with a significantly longer weapon can gain initiative.
Attacks from the rear prevent the defender from striking back on the first turn, and cause the defender to automatically strike last on the second turn. (It may be appropriate to note here that the rules actually say 'round', although 'turns' are referenced in the mass combat system. Nowhere is a distinction made between these in Chainmail, and a 'turn' is defined to be about one minute of battlefield time, making it the same as an OD&D 'round'.) Attacks from the left flank cause the defender to strike last on the first turn of melee.
After the first turn, whoever struck first before continues to strike first (static initiative!) unless their opponent has a weapon two classes higher or is above them. Of course, since the one initiating combat strikes first unless one of these two is true, this basically boils down to "if the defender automatically lost initiative because of being attacked from the rear or flank, reassess initiative once their forced loss ends" plus "if movement in combat places the defender above the attacker, the defender then gains initiative".
Weapon class comes into play in some more interesting ways as well. This gets complicated, so I'm going to quote:
"4) a. For any weapon 2 or more classes higher than the attacker the ability to parry does not exist.A couple of paragraphs later, we have: "A man wielding a weapon four classes lower (1 vs. 5, 2 vs. 6, and so on) strikes two blows during every melee round. If a man has a weapon eight classes lower, he will strike three blows during every melee round."
b. For any weapon 1 class higher to three classes lower than the attacker the defender may parry the blow by subtracting 2 from the attacker's roll, but he has no counter blow.
c. For any defender whose weapon is four to seven classes lower than the attacker, the defender has the option to give the first blow OR parry the attacker's blow, by subtracting 2 from the attacker's roll. If the attacker equals the original requirement for a kill the higher weapon breaks the defender's weapon. If the parry is successful, the defender gets one counter blow.
d. For any weapon whose class is eight or more classes lower than the attacker, the defender gets the first blow and may parry the second or strike the second. He subtracts one for the parry and a roll equal to the original kill requirement breaks the weapon. (Pikes, spears or lances of the attacker do get the first blow over lower class weapons if there is a charge. Here the length of the weapon prevents the defender, even with his lighter weapon, the ability to get the first blow.)"
Combined with the above, we can put this into chart form, using A for the attacker's weapon class, and D for the defender's:
D >= A+2: defender strikes first, but cannot parry
D <= A-4: defender strikes first and gets two attacks, but may give up the first one to parry, giving the attacker a -2 on their attack. If they do so and fail to parry, they lose their chance to attack at all.
D <= A-8: defender gets three attacks, with the order of attacks going D, D, A, D. The defender may give up their second attack to parry, giving their attacker a -1 on their attack; it's unclear whether they still get the third attack in this case.
otherwise: defender may choose to give up their attack in order to parry, giving the attacker a -2 on their attack.
For further edification, here are the weapons and their classes:
2. Hand Axe
5. Battle Axe
9. Halberd or other Polearm
10. Two-Handed Sword
11. Mounted Lance
Those who are intimately familiar with first edition AD&D may be noticing something here: namely, the "weapon classes" here are fairly similar to the "weapon speed factors" of that version of D&D! The specific rules even bear some similarities (especially the use of 4 and 8 factors difference as "key points"), although AD&D1 only brings weapon speed in when initiative is otherwise tied.
This is a fairly sophisticated system overall, taking in weapon length and speed in an interesting way, and giving an option of fighting defensively through parrying. Small weapons are very fast, but are also less effective at parrying more massive weapons. Longer weapons can give the first strike to a defender, but are also slower, which can give reason for carrying multiple weapons, such as a polearm or spear wielder carrying a dagger or sword. The possibility for lighter weapons to break on a parry also encourages this.
In D&D, this system was the one originally meant to be used. In light of that, certain things about original D&D make more sense:
First, adjustments to attacks were small. A character could gain a +1 or -1 to missile attacks from Dexterity, but that was it. On 2d6, however, that +1 or -1 is much more significant than with 1d20! A +1 magical weapon was quite powerful, and the largest bonus from a magical weapon in OD&D was +3 (and bonuses this high were almost exclusively against a particular opponent type, such as a Sword
Upon advancing a level, the rules did not make it clear how new hit points were to be established. Would a magic-user advancing from level 1 to 2 simply add 1 hit point to their existing total? For advancing from 2 to 3, should they then remove that 1 hit point and add the result of a die roll in place? These would seem to have some logic, but, from the evidence of modern re-creations such as Swords & Wizardry and Delving Deeper, it would seem that common practice was to re-roll hit points at each level completely, but continue to use the old hit point total if the new one turned out to be lower.
A 'hit' in combat did 1d6 hit points of damage instead of scoring a kill - hence one's hit dice was approximately how many hits it would take to kill them, on average (excepting those with high or low Constitution adjusting their hit points per die). Speaking of that, as with attack adjustments, hit point adjustments for Constitution were kept small - no more than +1 or -1 per die.
The familiar d20-based system for attacks was present in OD&D, but was marked as the "alternative combat system", for those who did not have Chainmail. Here we see the familiar D&D armor class system, with AC 9 being "no armor or shield", and AC 2 "plate armor & shield". In spite of being "alternative", though, this quickly became established as the standard D&D combat system, and is the system used in all subsequent iterations of D&D.
Speculation here: I would guess that D&D's sales well outstripped those of Chainmail, and thus, most players were actually using the "alternative" system. Indeed, in an FAQ for D&D presented in issue 2 of TSR's official newsletter The Strategic Review, it is said that "it is suggested that the alternate system in D&D be used to resolve the important melees where principal figures are concerned, as well as those involving the strong monsters". Thus, at this point ("Summer 1975", TSR being a quarterly publication), the 'alternative' system had essentially become the standard system. This issue also presented an initiative system, which was the simple "roll 1d6 for each combatant and add dexterity bonus; re-roll each round". This system has changed often over the course of D&D's lifespan, so we'll be returning to it many times in this series.
There is also an extensive combat example given in that FAQ, which we will return to in the next column, when we fully dive into OD&D, starting with looking at the original game's classes!
Gygax, Gary and Arneson, David, Dungeons & Dragons. 1st edition, Tactical Studies Rules, 1974.
Gygax, Gary and Perren, Jeff, Chainmail, 3rd edition, Tactical Studies Rules, 1975.