"A helmet is usually thought to be very heavy, but when one is attacking a castle or something similar, and arrows, bullets, large rocks, great pieces of wood, and the like are coming down, it will not seem the least bit so."Helmets have been around since the literal beginning of D&D as a commercially available product, but their actual use in play has never been straightforward. I won't go into detail on the rules for helmets tucked away in an obscure paragraph of the 1st Edition Dungeon Masters Guide, but instead I'll focus on the edition in which I've logged the most designing time (refereeing time is a close tie with B/X): AD&D 2nd Edition.
- from the 11th chapter of Hagakure
The Player's Handbook for 2e has two types of helmet listed: the basinet and the great helm. Their use or employment is never described, even though the designers thought that the people actually reading the rules would need an explanation of what a magnifying glass does. As much as I want to keep things in the core books whenever possible (to cut down on the strain on my back, if for no other reason), the solution is found in supplements.
The Complete Fighter's Handbook (PHBR1) gives several types of helmets, but doesn't provide specific rules as to what helms should be used with what armor. I realize that such is a task I could undertake myself, but there's little point in doing so if someone else has already done it. In Player's Option: Combat and Tactics, there are more specific rules for different helmets, along with corresponding Armor Class values.
In my current house rules, I use helmets a little bit differently (though still heavily inspired by both of those books), so I'll provide the pertinent rules below.
Fuzzy's Helmet Rules for AD&D 2nd Edition
AC 8-7 (padded, leather, studded leather, ring mail): Leather helm
AC 6 (brigandine, scale mail, hide): Cap
AC 5 (chain mail): Mail coif
AC 4 (splint mail, banded mail, bronze plate): Open-faced helm
AC 3-2 (plate mail, field plate): Closed-face helm
AC 1 (full plate): Great helm
A helmet can be worn with different armor (i.e., wearing a great helm with bronze plate), although in some cases this will look off-putting to observers, though not usually resulting in any penalty to interaction. A mail coif can be worn underneath a great helm; this does not provide a bonus to protection, but still allows some protection for the head if the great helm must be removed.
The main function of helmets is to prevent called shots being made to the head. If the head is unarmored, it has AC 10, and a successful called shot will deal the maximum possible damage for that weapon (so a long sword against a Medium opponent would deal 8 points of damage, plus Strength bonus if applicable).
Unofficial Rule (need to test this out in play first): A natural 20 on a called shot to the head will result in the target being instantly reduced to 0 hit points, thereby falling unconscious and "hovering on death's door".