DMG grappling converted to "attack roll vs AC"

DMG grappling, in brief 

Most of the grappling and overbearing systems presented in early D&D (namely, those found in Strategic Review Vol. 1, No. 2 and Unearthed Arcana) use a conventional attack roll, while the AD&D DMG (pp. 72-73) infamously uses a separate system using percentile dice. To make sense of the DMG system in my own mind, I converted the grappling system to a more conventional looking system with an attack roll vs AC.

The DMG system actually has two percentile rolls, first a "Base Score to Grapple" roll (which I have converted to an attack roll vs AC) and second a "Hold Determination" roll (which is similar to the damage roll).

Interestingly, the DMG system differs fundamentally from a normal attack roll in many ways:

- armour worsens, rather than improves, the AC of the defender (ie, it increases the Base Score to Grapple)

- the AC of the defender is not affected by having a shield or having any sort of magical armour or protection

- there is no attack bonus for high STR

- there is no progression of "to hit" number for higher level attackers (Yes, I am discounting the "Variable" described on p. 72 as even a 10th level fighter gets but +1 or +2 to the attack roll (6-11%).)

- wearing less armour gives the attacker a [b[tremendous[/b] "to hit" bonus

Converting the "Base Score to Grapple" to an attack roll vs AC

AC of defender, descending (ascending)

No armour: 0 (20)

Leather: 2 (18)

Chain: 4 (16)

Plate: 6 (14)

THAC0 ("to hit" bonus) of attacker by armour worn

No armour: THAC0 1 (+19)

Leather: THAC0 5 (+15)

Chain: THAC0 11 (+9)

Plate: THAC0 15 (+5)

In other words, a character or creature without armour is almost guaranteed to make a successful grappling attack regardless of the defender, while, for instance, a character in plate mail has to roll 15 or higher (30% chance) to disrupt an unarmoured spellcaster by a successful grappling attack.

In the combat example given in Strategic Review Vol. 1, No. 2, five orcs (AC 6, so presumably leather and shield) attack a 4th level fighter in chain and shield.

Using the DMG system, the five orcs in leather would have to drop their shields to make a grappling attempt and then roll 1 or higher to make a successful grappling attack against the fighter. However, the fighter would get a chance to fend off the orc attackers. It is not clear whether the fighter gets a chance to fend off each of the orcs. Assuming they do, then the fighter would need to roll a 10 or better to fend off each one (AC 7, 4th level fighter has a THAC0 of 17). Thus, each orc has a 45% chance to make a successful grappling attack, while in the Strategic Review system, each orc has to roll 15 (30% chance).

The higher percentage chance is offset by the fact that each of the successful orcs would need to make a "Hold Determination" roll, and they would get a -10% (-2 on a d20) when determining the hold because of the fighter's shield. However, the worse result is waist clinch (30% chance), which can be immediately countered, but there is little risk of the orc's being thrown off and stunned, unlike in the Strategic Review system.

30% chance of waist clinch, which can be immediately countered

20% chance of arm lock

15% chance of hand lock

15% chance of bear hug

15% chance of headlock

5% chance of strangle hold

To summarize, the DMG system is unique in that

- the attacking capability of the attacker is irrelevant to a successful grappling attack, but the attacking capability of the defender is highly relevant if, and only if, the defender has a weapon and is not surprised

- magical armour and shields play no roll in defence, and wearing armour actually negatively affects the defender

- a successful attack is primarily determined by the armour worn by the attacker and secondarily by the armour worn by the defender

- relative DEX and STR play a very minor roll

The "escape valve" of the system seems to be that a waist cinch is a weak hold that can be immediately countered by the defender; the escape valve should work if the defender is considerably larger than the attacker, but otherwise, it will be quite random.

Note 1: In the examples and percentages given above, I am simplifying by ignoring special cases where the defender is slowed, stunned or hasted, or faster speed. I am also ignoring the effect of DEX, STR, and relative weight and height.

DEX will typically give a bonus of +2-3 to the grappling "attack roll", but it will not differ much between characters.

Relative DEX and STR will have some effect on the "Hold Determination" roll, but it doesn't seem significant. If the attacker has 10 DEX and 10 STR, and the defender 18 DEX and 18/00 STR, the attacker would get a penalty of only -1 (-4%) on the "Hold Determination" roll. Compared with the penalty of -2 (-10%) if the defender has a shield.

Note 2: One more thought. There are various house rules regarding the appropriate AC for a "touch" attack. I like the idea of taking AC and subtracting the non-magical portion of armour other than shields, so -2 for leather, -5 for chain, -7 for plate. 

However, there is a case to be made for using the Base Score to Grapple roll for a touch attack, or at the least, for penalizing a defender for armour worn. Following the penalties in the grappling rules, you would take the full AC value with the appropriate penalty: -4 for leather, -9 for chain, -13 for plate. 

Note 3: I originally posted this on the Dragonsfoot 1st edition AD&D forum. To date, it is my only post to garner zero replies!