Armor, along with the use of a shield, is the basis for determination of how easily a character can be struck by an opponent’s weapon.
Other factors modify this, of course. Dexterity and magical effects are the two principal modifiers.
Do not confuse armor which is worn with the armor class (AC) rating of a monster. Although a creature might be given a very high armor class because of its exceptionally thick hide, armor plating, chitinous exoskeleton, or the like, other factors are considered in such ratings.
For example, the size of the creature, its speed, its agility, and perhaps its supernatural (extra-dimensional or multi-planed existence) aspects are considered in the armor class of all non-human type monsters. Therefore, a monster with an armor class of 2 will not be carrying a shield.
Armor types are given on the table below. Note that the inclusion of a shield raises armor class (AC) by a factor of 1 (5%), but that assumes attack from the front where the character can interpose it between himself and a blow.
The table assumes human size. The DM must determine cost and encumbrance of odd-sized armor. The encumbrance factor for armor does not consider weight alone; it also takes into account the distribution of the weight of the armor and the relative mobility of the individual wearing the protective material. Therefore, weights for armor shown are adjusted weights, and base movement speed is likewise shown. (2)
Move Rate Adjustment. The base movement speed of the creature will be modified downward based on bulk.
Donning or Removing Armor. The Dungeon Master must adjudicate the time necessary to complete this action based on the type of armor.
Armor Class is the basis for determination of how easily a character can be struck by an opponent.
Armor Rating
If you allow weapon type adjustments in your campaign, please be certain to remember that these adjustments are for weapons versus specific types of armor, not necessarily against actual armor class.
In most cases, monsters not wearing armor will not have any weapon type adjustment allowed, as monster armor class in such cases pertains to the size, shape, agility, speed, and/or magical nature of the creature.
Not excluded from this, for example, would be an iron golem. However, monsters with horny or bony armor might be classed as plate mail if you so decide, but do so on a case-by-case basis.
Naturally, monsters wearing armor will be subject to weapon type "to hit" adjustment.
Banded mail is a layered armor with padding, light chain, and series of overlapping bands of armor in vulnerable areas. Weight is somewhat distributed.
Chain mail is padding plus interlocking mesh armor covering the upper and lower body. Vulnerable areas have multiple thicknesses. Weight falls upon the shoulders and waist of the wearer.
Chain, elfin, is a finely wrought suit of chain which is of thinner links but stronger metal. It is obtainable only from elvenkind who do not sell it.
Leather armor is shaped cuir bouli (leather hardened by immersion in boiling oil) cuirass and shoulder pieces and softer shirt and leggings.
Padded armor is heavily padded, quilted coat and an additional soft leather jerkin and leggings.
Plate mail is light chain with pieces of plate - cuirass, shoulder pieces, elbow and knee guards, and greaves. Weight is well distributed.
Plate armor is a full suit of plate which is no more weighty and a bit less bulky, considering what is known as “field plate”. If the DM allows such armor in their campaign, use the same weight, with a 9” movement base and a base armor class of 2 sans shield. Such armor would be very expensive, c.2,000 g.p.
Ring mail is relatively soft leather armor over padding. To the long coat of leather are sewn metal rings. This makes the coat rather heavy and bulky.
Scale mail is armor similar to ring mail, but overlapping scales of metal are sewn to both coat and leggings-or a skirted coat is worn. As with chain, weight falls mainly on the wearer’s shoulders and waist.
Splint mail consists of light chain, greaves, and a leather coat into which are laminated vertical pieces of plate with shoulder guards.
Studded leather is leather armor to which have been fastened metal studding as additional protection, usually including an outer coat of fairly close-set studs (small plates).
When magic armor is worn, assume that its properties allow movement at the next higher base rate and that weight is cut by 50%.
Magic armor negates weight, so that movement does not consider any encumbrance from magic armor. There are otherwise various types of magic armor.
For game purposes all magical armor should be considered as being virtually weightless — equal to normal clothing, let us assume. This gives characters so clad a base movement speed equal to an unarmored man.
There is no magical elfin chain mail.
A shield is basically a barrier between its wielder and his or her opponent. It is used to catch blows or missiles. It can also be used offensively to strike or push an opponent. The shield can be used fully only to the left or front of the right handed individual. Attacks from the right flank or rear negate the benefits of a shield. [DMG, p.28]
A small shield can be counted against only one attack per melee round.
A normal-sized shield can effectively be counted against two attacks per melee round.
A large shield is counted against up to three attacks per melee round.
Shield, large, includes such shields as the large Viking round shields or the Norman kite shields. They are made of wood, covered with leather, and bordered with a soft iron banding at the edges. Although a large shield such as a Norman kite shield or a large Viking round shield covers much more of the body, employing one of these shields is far more difficult, as they are cumbersome and fatiguing. Therefore, large shields are treated as but +1 to armor class rating without a shield. Optionally, the DM may allow them to add +2 to this armor class rating with respect to small (non-war engine or giant hurled) missiles; if so, however, the DM will be certain that to also keep careful track of encumbrance.
Shield, small, is the typical kite and heater shields or small round shields constructed as a large shield, or else made of metal (more rare by far).
Shield, Buckler is a form of Small Shield that protects the user against but one opponent (of the user's choice) during any melee round in which the shield is employed. It is small and easily carried, and may be strapped to the arm, making it able to be used by crossbowmen and slingers. A small demi-human such as a halfling or a gnome would be fully protected by the buckler, and it is treated as a small shield in those cases. [UA, p.76]
Shield, small, wooden, is the same as other shields, but it lacks the metal binding and reinforcement, so it will be more easily split.
[Smaller] shields are less cumbersome and fatiguing in employment, however, so no distinction is made between a small and a normal-sized shield in AD&D.
Magic Shields. Magic shields are no less weighty than their non-magical counterparts, but they are non-bulky with respect to encumbrance.
It is assumed that an appropriate type of head armoring will be added to the suit of armor in order to allow uniform protection of the wearer.
Wearing of a “great helm” adds the appropriate weight and restricts vision to the front 60° only, but it gives the head AC 1.
If a helmet is not worn, one blow in 6 will strike at the AC 10 head, unless the opponent is intelligent, in which case 1 blow in 2 will be aimed at the AC 10 head (d6, 1-3 = head blow).