An oft lamented aspect of Dungeons & Dragons is that as characters advance in level they become better at attacking, but not at defending. Occasionally it is contended that this aspect is represented by increased hit points, and that their loss partially represents the gradual erosion of defences due to fatigue. This might be a sustainable point of view if hit points were easily replenished without recourse to magic, but they are not. A recovery rate of one or two hit points per day of rest is not particularly suggestive of fatigue. In the Chain Mail man to man melee system, the ability to parry is a function of weapon class; a combatant has the ability to parry so long as his weapon is of a class no more than one above that of his opponent. The parry takes the form of imposing a −2 penalty to hit in most cases, but if the difference between weapon classes is eight or more it becomes only a −1 penalty. In any case an attack must be given up for a parry to take effect, though a character may have up to three attacks if his weapon has a sufficiently lower class than that of his opponent.
Whilst these Chain Mail rules could be used with Original Dungeons & Dragons, an additional rule was introduced in the Greyhawk supplement, to the effect that fighters could use their dexterity to attempt to “dodge or parry opponents' attacks”; for every point above fourteen, they would impose a −1 penalty to hit on attacks against them. This is obviously similar in conception to the protection afforded by a shield. By contrast, the Blackmoor supplement took a different tack with the monk class. Prohibited from wearing armour, the monk received a defensive bonus expressed as armour class that increased as he gained in experience levels. However, more significant was his ability to parry or dodge missile attacks (even magic missile) by means of a successful saving throw. This was a paradigm shift conceptually from what had gone before, introducing a second die roll unrelated to the attack roll and capable of negating all damage. This defensive mechanism took no account of the fighting ability of the attacker, only the experience level of the defender.
The defensive capabilities of the monk made the transition to Advanced Dungeons & Dragons relatively unscathed, and the ability of the fighter to use his dexterity to defend himself was extended to all combatants. The parrying rules from Chain Mail, though, were incorporated only partially, so that a character could impose a penalty on enemy attacks equal to his strength bonus at the expense of making any attacks that round. This was obviously of little use to a character with a strength less than seventeen. The cavalier class was introduced in Dragon #72, able to parry “more effectively” so that all of his bonuses to hit could be used as a penalty against enemy attacks. This class could also make a parry against a second opponent using his shield, which imposed a penalty equal to its defensive value, but prevented it from being used for the rest of the round. This was a long way from the original concept as articulated in Chain Mail, being over reliant on high attribute scores, as well as bonuses from magical weapons and armour.
With the second edition of Advanced Dungeons & Dragons, an optional parrying rule was introduced as a function of level and class. A character could give up his chance at action for the round to gain a bonus to armour class against melee attacks equal to half his level, with an additional +1 for fighters. That same year, the Complete Fighter’s Handbook suggested a second option, where a character could give up an attack to attempt a parry. This required the character to hit his opponent’s armour class to negate the attack, inadvisedly implying that the armour class of a combatant was reflective of his fighting ability. When the rule was revised and incorporated into Combat & Tactics, this premise was wisely discarded and a fixed armour class set as a level of difficulty. This should all sound familiar to players of War Hammer, where a character can similarly trade his attacks for parries, needing to roll under his weapon skill to reduce damage by 1d6 in first edition; in second edition all damage is negated on a successful parry.
A noticeable problem with Advanced Dungeons & Dragons second edition is its approach to fighting with two weapons. Even though it limits the number of secondary attacks to only ever one, this still effectively doubles the attacks available to most characters. Coupled with the potential to eliminate the normal penalties through high attributes or proficiency slots, and weighed against the relatively small benefits of using a shield, it is often viewed as something of a rules exploitation. This was probably why Gygax earlier specified that the “secondary weapon does not act as a shield or parrying device in any event.” By contrast, War Hammer avoids this problem by not granting any additional attacks from secondary weapons; they are useful primarily because they provide bonuses to parry rolls. In developing Astonishing Swordsmen & Sorcerers of Hyperborea, we have had to consider carefully how any combat actions we suggest might interact with multiple attacks in order to ensure that no undue or unintended advantages are unexpectedly revealed during play.
Regardless of whether a parry, dodge, or block takes the form of an attack penalty or a saving throw, the action must be traded against a commensurate disadvantage. A simple trade off of fighting ability against armour class is a fairly straightforward approach, amounting to –X to hit in return for –Y to be hit. However, the benefit of this trade depends on the number of attacks the character is exposed to in a round. The obvious recourse is to trade against the number of attacks the character has available, but this may unduly enhance the benefits of a secondary weapon. Since a shield lowers effective armour class by one, a secondary weapon ought to be of no greater advantage. Other methods to consider for modelling a parry, dodge, or block include opposed rolls, parallel armour class, and forced rerolls. In those cases, shields would have to be treated exactly like secondary weapons, meaning attack penalties for the primary weapon. I have made available a selection of possibilities for your perusal here and a compilation of the rules mentioned above here.