Parrying (Optional Rule) (DMG)

During a one-minute combat round, each character is assumed to block many attempted attacks and see many of his own attacks blocked. In normal combat, characters parry all the time—there's no need to single out each parry.
When a character deliberately chooses not to parry, his chance of being hit increases. A mage casting a spell, for instance, gains no AC adjustment for Dexterity. Thus, choosing to parry, in and of itself, is not a separate option under the AD&D game rules.
At the same time, the assumption is that characters in combat are constantly exposing themselves to some risk—trying to get a clear view of a target or looking for the opening to make an attack. There are times, however, when this is not the case. Sometimes, the only thing a character wants to do is avoid being hit.
To make himself harder to hit, a character can parry—forfeit all actions for the round. He can't attack, move, or cast spells. This frees the character to concentrate solely on defense. At this point, all characters but warriors gain an AC bonus equal to half their level. A 6th-level wizard would have a +3 bonus to his AC (lowering his AC by 3). A warrior gets a bonus equal to half his level plus one. A 6th-level fighter would gain a +4 AC bonus.
This benefit is not a perfect all-around defense, and it's not effective against rear or missile attacks. It applies only to those foes attacking the defender from the front. This optional defense has no effect against magical attacks, so it wouldn't do anything to protect a character from the force of lightning bolt or fireball spells.