The mysterious "Alternative Combat System" first mentioned in OD&D Vol.1 "Men & Magic" appears to have caused much confusion over the last 50 years. Especially regarding its connection with the combat rules in its predecessor, CHAINMAIL. It appears to me that for these past 5 decades there has been a bit of misinterpretation in regards to utilising the Alternative Combat System in the way it was initially intended. This if fine, of course, nobody needs to, or even should play the original Dungeons & Dragons game rules as written. In many ways, you cannot. As the rules were supposedly left incomplete in purpose to allow for variation and evolution. However, like most things, in order to get the best of of it, making well-informed decisions in its regard, an adequate understanding is a huge boon!

OD&D Alternative Combat System, what is it and when should you use it?

What is commonly known about the Alternative Combat System is supplemented by an article Gary Gygax wrote in Strategic Review issue #2, which, basically, was a Dungeons & Dragons F.A.Q. to aid early players of the game with their understanding of certain aspects of the rules. The article sheds some light on the intended utilisation of the Alternative Combat System. It explains that:

"Combat: CHAINMAIL is primarily a system for 1:20 combat, although it provides a basic understanding for man-to-man fighting also. The Man-To-Man and Fantasy Supplement sections of Chainmail provide systems for table-top actions of small size. The regular CHAINMAIL system is for larger actions where man-like types are mainly involved, i.e. kobolds, goblins, dwarves, orcs, elves, men, hobgoblins, etc. 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 stronger monsters. When fantastic combat is taking place there is normally only one exchange of attacks per round, and unless the rules state otherwise, a six-sided die is used to determine how many hit points damage is sustained when an attack succeeds. Weapon type is not considered, save where magical weapons are concerned. A super hero, for example, would attack eight times only if he were fighting normal men (or creatures basically that strength, i.e., kobolds, goblins, gnomes, dwarves, and so on)."

These paragraphs of text imply that depending on the situation, different methods of combat resolution should be used. We have:

The Man-To-Man system in CHAINMAIL.

The regular CHAINMAIL 1:20 system for larger actions where man-like types are mainly involved.

Finally, the Alternative Combat System for melees with principle figures and stronger monsters, i.e. Fantastic Combat

To add some clarity regarding when these three systems should be used we need to understand some common terminology used throughout the OD&D lifespan.

Man-like-types, which are also often referred to as Normal Types, means figures of less than 2 Hit Dice, i.e. figures who are not classed as being equivalent to more than a single person in their fighting capability.

Principle figures and stronger monsters are often referred to as Fantastic types, which means figures of more than 2 Hit Dice, i.e. figures who are equivalent to more than a single person in their fighting capability.

Once we understand these simple definitions we can much more easily make sense of the various OD&D combat systems and when they should be applied.

Now, there is a bit of confusion over what it means in OD&D when we talk about “Fantastic Combat”, some people presume that Fantastic Combat, which is when creatures of more than 2 Hit Die are fighting, should be conducted on the Fantasy Combat Table in CHAINMAIL. But actually... this is when the Alternative Combat System should be used, that is, the Attack Matrices that were given in Men & Magic. These attack Matrices were not supposed to replace the Man-to-Man system in CHAINMAIL, they were supposed to replace the Fantasy Combat Table, so that it encompasses all monsters and characters.

Let's look at the passage in Strategic Review issue 2, again, which says “It is suggested that the ALTERNATIVE SYSTEM in D & D be used to resolve the important melees where PRINCIPLE FIGURES are concerned, as well as those involving the STRONGER MONSTERS. When fantastic combat is taking place there is normally only one exchange of attacks per round, and unless the rules state otherwise, a six-sided die is used to determine how many hit points damage is sustained when an attack succeeds. Weapon type is not considered, save where magical weapons are concerned. A super hero, for example, would attack eight times only if he were fighting normal men (or creatures basically that strength, i.e., kobolds, goblins, gnomes, dwarves, and so on)."

Further, in the type-written draft of OD&D, it says “Fantasy vs. Fantasy: Because of the vast numbers of fantastic creatures and levels of men, a matrix to show hits is totally impractical (we know because we tried, but with 70 or more categories it becomes too unwieldly to handle). We therefore recommend that a system utilising a 20-sided die (increments of 5%) be adopted.” The draft then goes on to provide the Attack Matrix.

The draft also explains that  if normal types are fighting fantastic types then the 1:20 Combat Tables should be used, with the normal types scoring 1 hit point when they hit and the stronger creatures scoring hits equal to a die roll (1-6).

So in summary: If creatures of more than 2 hit dice are fighting creatures of more than 2 hit dice (fantastic types) Use the ALTERNATIVE COMBAT SYSTEM ATTACK MATRICES. There is normally only one exchange of attacks per round, and unless the rules state otherwise, a six-sided die is used to determine how many hit points damage is sustained when an attack succeeds. Weapon type is not considered, save where magical weapons are concerned.

Another term that is sometimes dropped into the various OD&D "scriptures" is "Super-Normal Types" and this basically means figures who can advance upwards in levels, i.e. player characters. These could also be considered as principle figures and therefore could be also be considered as Fantastic Types, but I think this is open to discussion. Perhaps they are Fantastic Types when they achieve a fighting  capability of 2 or more Hit Dice.

Men & Magic Attack Matrix 1.: Men Attacking

All of the above can be faithfully boiled down to the following guidelines for deciding which combat system to use:

NORMAL TYPES VS. NORMAL TYPES

If small numbers of creatures of less than 2 hit dice (normal types) are fighting it will be most interesting to use the Man-to-Man Melee Table.

If large numbers of creatures of less than 2 hit dice are fighting the Combat Tables based on a troop ratio of 1:20 should be used.

NORMAL TYPES VS. FANTASTIC TYPES

If creatures of less than 2 hit dice (normal types) are fighting creatures of more than 2 hit dice (fantastic types) then the 1:20 Combat Tables should be used, with the less than 2 hit dice creatures scoring 1 hit point when they hit and the stronger creatures scoring hits equal to a die roll (1-6). Of course some creatures cannot be hit by normal types, and no hit points will be scored against them.

FANTASTIC TYPES VS. FANTASTIC TYPE

If creatures of more than 2 hit dice are fighting creatures of more than 2 hit dice (fantastic types) Use the ALTERNATIVE COMBAT SYSTEM ATTACK MATRICES. There is normally only one exchange of attacks per round, and unless the rules state otherwise, a six-sided die is used to determine how many hit points damage is sustained when an attack succeeds. Weapon type is not considered, save where magical weapons are concerned.