The AD&D Monster Manual occupies an interesting space in D&D history. Interesting because it was published years before the rules for the “Advanced” game itself. Mike Carr’s 1977 introduction – only 3 years after D&D was published and roughly contemporary with Arnesons publication of the FFC – mentions D&D 13 times to AD&D’s 4 times.
Its been observed that the MM falls somewhere between OD&D and AD&D in conformity to the rules, and many regard it as an OD&D supplement with a few extras and indeed, for several years until the publication of the AD&D players handbook, it effectively was.
Mostly, that’s just an artifact of curiosity, but there exists on page five and the top of page 6 a mostly forgotten and rarely sited section called “Explanatory Notes” by Gary Gygax. This little section has information not found in other OD&D or AD&D books (excepting Dieties and Demigods, where it is largely repeated verbatim), that goes a long way to clarifying the intentions behind a number of the OD&D monster stats that directly relate to in game play choices. In Particular:
FREQUENCY: Now this is an interesting stat. For example, we are told a rare creature has an 11% chance of appearing in an “area or region”. This seems to be unused information. I suppose you could look at the tables of all the creatures that might be in a given kind of area – mountains lets say – roll percentiles repeatedly until you had narrowed the creatures to one “frequency” type - Rare lets say - and then roll against the list of rare creatures to find the one encountered. I don’t think anybody really did this nor do I get the sense that the random encounter tables in the DMG are really keyed with this stat in mind (although they could be, I dunno).
NUMBER APPEARING: Not explained. We are told it is an “average” “guideline””to be altered … as the need arises”. We are also told “It is not generally recommended for use in establishing the population of Dungeon Levels.”
So, what does it represent? Well, looking back to Frequency, we could safely assume the number appearing is the population in “a region or area where it might be an inhabitant”. This could mean a “region” of a dungeon level, presumably but not usually an entire level.
The alternative meaning is that Number Appearing is how many monsters would be randomly encountered if PCs stumbled onto them as wandering monsters. This is how many have understood it.
Hmm, lets look a little further and see if there are more clues.
% IN LAIR: We’ve looked at this one back in August, but here is something else interesting “…where it domiciles and stores its Treasure. If a monster encountered is not in its lair it will not have any treasure unless it carries “individual” treasure…”
Which ties in to:
TREASURE: these are the types listed on a table which can appear with a monster listing. About these types Gygax writes that they “… are only found in lairs of monsters as found above.”; and “The use of treasure type to determine the treasure guarded by a creature in a dungeon is not generally recommended.”
Frequency covers “an area or region”
Treasure Types are meant for the domicile (Lair) in an area or region, not an individual creature and not as an isolated dungeon treasure of the sort used when placing a monster in a room.
Number Appearing should represent the creatures who will have the assigned treasure type in an area or region. Otherwise, there is a guide given for region, a lair, and treasure to be found, but none for how many inhabitants may be found there. If somehow the Number Appearing was not meant to function in harmony with the other stats but referred to wandering, out of lair encounters it would make more sense to list individual treasures for the wanderers, not lair treasures for the unknown number of the “domicile” population.