Over at Delta’s D&D Hotspot, there was a discussion of the parry rule in AD&D. In general, this is a pretty useless rule because a character gives up their attack for a bonus to AC. (Well, it’s a penalty to the opponent’s “to hit” roll, but we’ll consider that equivalent.) In D&D combat, if you aren’t attacking, you’re conceding. But special circumstances can arise where this tactic does make sense.
AD&D compounds the uselessness of the rule, however, but setting the bonus equal to the characters “to hit” bonus due to strength.
Chainmail has a parry rule as well. Though it is complicated by its weapon classes rules. The weapon class rules are reasonable, but more complexity than I’d want. The important bit is that Chainmail makes the benefit a flat +2.
Dan also makes a good argument that a +1 in Chainmail should become a +2 in D&D.
It turns out that the d20 SRD has a similar total defense rule, which gives a +4 to AC.
So, that’s what I’m going with...
If a character chooses total defense, they may not attack that round, but they gain a 4-point bonus to AC.