Among the more infamous components is the elf-only multiclassing (as we now call it) rule. Today let's read it with a perfectly open mind. Here in its entirety is the description for elven characters (Vol. I, p. 8):
Elves: Elves can begin as either Fighting-Men or Magic-Users and freely switch class whenever they choose, from adventure to adventure, but not during the course of a single game. Thus, they gain the benefits of both classes and may use both weaponry and spells. They may use magic armor and still act as Magic-Users. However, they may not progress beyond 4th level Fighting-Man (Hero) nor 8th level Magic-User (Warlock). Elves are more able to note secret and hidden doors. They also gain the advantages noted in the CHAINMAIL rules when fighting certain fantastic creatures. Finally, Elves are able to speak the languages of Orcs, Hobgoblins, and Gnolls in addition to their own (Elvish) and the other usual tongues.
Now, what does this "freely switch class" possibly mean? First let's focus on the benefits from each class. It's called out that they can use armor (magic only?) as both Fighters and Magic-Users; apparently other benefits are exclusive. So, it looks like they only gain higher hit dice, attacks, saves, and martial weapons when in "Fighter-mode". And apparently they can only access spells and general magic-item usage in "Magic-mode".
What are the implications of a split like this? Does it mean that memorizing magic spells enervates the physical strength represented by hit dice, attack rolls, and saving throws? Does touching a martial weapon's material drain the knowledge of magic spells from one's mind?
But more importantly, what exactly does it mean to "switch" classes? In this era, we're used to characters having multiple classes, each with its own XP bank (AD&D) or at least level (d20 System). This text, however, doesn't read that way. Let's say we take the phrase "switch" at its most literal. Perhaps a character has a fixed XP value, and before each adventure can actually decide between one of two character tables to apply that XP towards. In any particular adventure, you get the sum total of XP applied in its entirety towards one class.
In some ways that would be more elegant -- No need to keep multiple XP or level totals. No need for any additions or compositions of class abilities. No tracking separate level totals. One value for XP, applied to one single class in any particular adventure. And again this looks most compatible with the language on p. 10: not "multiclassing" or "dual-classing" used there, but again the specific phrase "changing class":
Changing Character Class: While changing class (for other than elves) is not recommended, the following rule should be applied: In order for men to change class they must have a score of 1 6 or better in the prime requisite (see below) of the class they wish to change to, and this score must be unmodified. A Cleric with a "strength" of 15, for example, could not become a Fighting-Man. In any event Magic-Users cannot become Clerics and vice-versa.An interesting interpretation, I think, with intriguing implications for the campaign-world of wizards and how their magic functions. Unfortunately, this particular reading is still entirely incompatible with the text written for elves encountered as monsters in the same ruleset (Vol. II, p. 16):
ELVES: Elves are of two general sorts, those who make their homes in woodlands and those who seek the remote meadowlands. For every 50 Elves encountered there
will be one of above-normal capabilities. Roll a four-sided die for level of fighting and a six-sided die for level of magical ability, treating any 1's rolled as 2's and 6's (magical level), as 5's. For every 100 encountered there will be a Hero/Warlock...
You can see here that NPC elves (at least) are given two specific, different levels in their classes of Fighting and Magic-Use. And no mention is made of any need to have selected which class is functional when they are encountered in the wild.
So, once again, we are thrown to our own devices when refereeing OD&D out of the original box. Something must give to preserve logical consistency, and the phrase "freely switch class" is the most likely sacrificial victim. Let a thousand multiclassing rules proliferate...