Players Handbook (Revised), p. 42, top of 2nd column:
"Wizards cannot wear any armor, for several reasons. Firstly, most spells require complicated gestures and odd posturings by the caster and armor restricts the wearer's ability to do these properly. Secondly, the wizard spent his youth (and will spend most of his life) learning arcane languages, poring through old books, and practicing his spells. This leaves no time for learning other things (like how to wear armor properly and use it effectively). if the wizard had spent his time learning about armor, he would not have even the meager skills and powers he begins with. There are even unfounded theories that claim the materials in most armors disrupe the delicate fabric of a spell as it fathers energy; the two cannot exist side by side in harmony. While this idea is popular with the common people, true wizards know this is simply not true. If it were, how would they ever be able to cast spells requiring iron braziers or metal bowls? ... A wizard...cannot use magical armor, because no armor is allowed."
As for multi-classed characters:
ibid., p. 62:
"A multi-classed wizard can freely combine the powers of the wizard with any other class allowed, although the wearing of armor is restricted. Elves wearing elven chain can cast spells in armor, as magic is part of the nature of elves."
ibid., p. 128 (bottom of right-hand column):
"If a character is attempting to wrestle in armor, the modifiers on Table 57 are used." [Table 57: studded leather -1 through full plate -10]
"Once these restrictions are lifted, the character must abide by the restrictions of whichever class he is using at the moment. A dual-class fighter-mage, for example, cannot cast spells while wearing armor."
PH2R, p. 113, last paragraph in gray box:
"Spell Components (Optional Rule)...The three different aspects of spell components also change the conditions under which your character can cast his spells. No longer does he need to be able to speak, move, and use some item. He only needs to fulfill the required components. Thus, a spell with only a verbal component could be used by a naked, bound spellcaster. One requiring only gestures could be cast even within the radius of a silence spell. Most spells require a combination of components, but clever spellcasters often create new spells that need only a word or a gesture, enabling them to take their enemies by surprise."
Player's Option: Skills & Powers, p. 61 (and again on p. 62):
"Armored wizard (15): This wizard can cast spells while wearing the armor of his choice."
Player's Option: Spells & Magic, p. 24:
"Armor (5/10/15): With this ability, a wizard may ignore the normal restriction against using armor. As a 5-point ability, the wizard may wear padded armor; for 10 points, the wizard may wear leather, studded leather, hide, or brigandine armor; and for 15 points, the wizard may wear any armor he chooses. Note that a wizard may not use any kind of shield."
ibid., p. 46:
"Restrictions: Multi-classed wizards gain the full benefits of all their classes, but must abide by any restrictions that aren't specifially negated by a class benefit....While a multi-classed fighter/wizard may wear any armor he chooses, the armor itself interferes with the character's ability to cast spells. In order to cast a spell, the character must remove his armor. The only kinds of armor that do not interefere with spellcasting are elven chain mail and elven plate mail. (Note that single-classed wizards can't use these special armors because they're not trained in the wearing of armor.)
"Optional Powers: With the DM's permission, multi-classed wizards may select some of the optional abilities described in Chapter 2, incliding armor,..."
The PH descriptions are for "standard" mages, rather than the exceptions to the rules. The standard mage (and standard specialist) had no time to learn the ways of warriors, as he was probably apprenticed to an experienced mage very early on. The stereotypical mage is obsessed with studying magic, and thus spurns anything--such as armor that prevents certain movements--that might cut deeply into his spell repertoire, since *every* spell involves some somatic component. Armor is designed to allow the movements necessary to, for example, swing a sword and dodge attacks; there is not enough freedom of movement to allow the sorts of motions that are required for spell casting. The modifiers to attempting to wrestle in armor prove that armor does indeed interfere with free movement of the limbs.
I suspect many people differ on the AC benefits of someone utterly clueless in the ways of armor and used to just using innate agility (or magical ability) to dodge attacks donning a suit of armor; this section seems to imply that they are nil or possibly that any AC benefits are negated by the wearer's lack of knowledge not to do things like raise one's arms to cover one's face and thus expose the armpits when defending against a swordsman or just stand there in the face of the imminent attack with faith that the armor is all-protecting--which, in a system that deals with a combat round that involves dodging, parrying, attacking, tiring, etc. all in one roll and one AC#, can be deadly reactions. However, it is written elsewhere in the Players Handbook thieves may don any armor at the expense of losing some of their thief abilities for the duration. This can be explained (though not suitably to some) by figuring that, since thief training covers some armor use (leather/studded leather), it also covers enough of armor use in general to apply some knowledge to any armor; whereas the standard wizard knows next to nothing about armor, and thus has nothing to apply. The spell disruption problem is enough to drive most mages to spurn armor completely, and the utter lack of any protective bonus convinces the rest to not bother.
Since the problem of mages wearing armor apparently involves *both* restriction of movement *and* lack of training to get any meaningful bonus out of the armor, both givens have to he overcome for a mage to wear armor.
A fighter/mage dual-class (assuming the levels work out so that class-ability restrictions are lifted) can wear armor, or cast spells, but not both at the same time. The fighter/mage has had training in the use of armor, but only in the context of being a warrior; mage training was a completely separate set of studies. The fighter/mage cannot overcome the movement problems inherent in wearing armor, and thus cannot cast spells while wearing armor.
A figher/mage multi-class has a mish-mash of training in both fields. From elves' ability to cast spells in special types of armor, and the dual-class abilities in general to simultaneously use abilities of both classes one can infer that "the wearing of armor is restricted" actually means "restricted only when the Fighter/Mage desires to use certain mage abilities and not be just a fighter" rather than "the only type of Fighter/Mage that can wear armor is an elf, and even then only when the armor is elven chain." (An individual DM might use the latter interpretation however; this one is a judgement call--logical as the former assumption may be--and thus this can't really be considered part of the "default" rules.)
So far, everything in the "default" rules is self-consistent, assuming you accept certain givens, namely:
One interesting side-effect of all this is that if one has captured an enemy mage, the best way to leave him mobile (and not requiring a fireman's carry to transport him to the nearest jail) yet completely cut off his spell-casting ability is to take the time to strap him into a spare suit of armor. He will not even get any real AC benefits from the armor, so the situation is doubly humiliating! In addition, since every spell has a somatic, verbal, and material component, mages cannot cast any spells when tied up (see #5 below), gagged, or naked in a competely empty room.
Now to toss in optional & house rules:
WSG (1st ed.), p. 20:To be consistent with the way armor is treated, the above should read that any DM choosing to give the AC bonus should also deny spell use, since it is the bulk of the clothing that both gives the AC and also restricts movement. This goes for any bulky or restrictive conditions, from being tied up to very heavy ceremonial/traditional robes to parkas to armor. Ramifications: None, really; this merely deals with an eventuality that is not discussed in the current rulebooks and applies the "default" standards consistently to the new situation.
"[V]ery cold clothing is thick enough so that (at the Dungeon Master's discretion) it may be considered to impart an armor class of 8 to its wearer. SImilarly, cold clothing is the equivalent of armor class 9. Obviously this distinction is only important to character who are not wearint armor over the clothing--and to magid-users and illusionists, who are forbidden to wear actual armor of any sort. But, techinically, cold-weather clothing is /not/ armor, even though it does procide some protection from physical attacks. To balance the benefit that magic-users and illusionists can gain from being bundled up, the Dungeon Master may wish to rule that the wearing of thick, bulky clothing inhibits the character's ability to move freely. This could be translated into (for instance) a base chance of 10% that any spell requiring a somatic component will be miscast adn thus ruined."
House rules that are not as universally implemented as the first included above, and aren't logical extensions of existing rules into uncharted territory as the other, but stay with the spirit of the rest of this article and create exceptions to the rule rather than rewrite the rule completely: