Hit Dice Notation

[Matthew]

Right from the original version of Dungeons & Dragons there is the use of X+Y as a hit die notation. It seems likely to me that it is rooted in the "Man+1" combat capability of a level one fighter, which is why subsequently normal men were given THAC0 20 contrasting with first level fighting-men having THAC0 19. In Monsters & Treasures it is noted that a troll with HD 6+3 can make six attacks against normal men and add three to one of them. Berserkers are noted as being HD 1+1, but add two when fighting normal men. At this early stage all hit dice are 1d6, so kobolds have 1-3 hp, goblins 1-5, orcs 1-6, and hobgoblins 2-7. With Greyhawk fighters get 1d8 hit dice, which has the same average as 1+1, but so do all hit dice move up.

With AD&D, fighter hit dice move up to 1d10 (av. 5.5), but for some reason men move down to 1d6 (av. 3.5), or in the case of mercenaries in the DMG 1d4+3 (av. 5.5). A lot of monsters have the same plus as their hit dice [e.g. 4+4], but others have a larger or smaller plus [e.g. 2+1 or 1+2]. The DMG indicates that any plus above three should be treated as an extra hit die for purposes of the attack matrices, but for the purposes of saving throws plus one to plus four should be treated as one extra hit dice, and plus five to plus eight as two extra, and so on. Many monster entries have less than a full hit die and are written variously as:

¼ [e.g. Giant Centipede]
½ [e.g. Brownie]
1 hit point [e.g. Brain Mole]
1-4 hit points [e.g. Kobold]
2-5 hit points [e.g. Leprechaun]
1-6 hit points [e.g. Bandit]
1-7 hit points [e.g. Goblin]
2-7 hit points [e.g. Berserker]
1−1 [e.g. Goblin in the DMG Appendices]

What I want to argue here is that each "plus" can be read as +¼ HD, so HD 3+1 = HD 3¼, and that for saving throws you round up, and for the attack matrices you round down [e.g. HD 3+3 = HD 3¾ = HD 3]. Obviously, at HD X+8 and above this latter does not quite work out, but such high pluses are fairly uncommon [i.e. Type VI Demons and Shedu].

There is also the matter of the numerous X+X monsters, such as elves (1+1), war dogs (2+2), giant water spiders (3+3), worgs (4+4), horned devils (5+5), and so on. It occurs to me that where the number of pluses is equal to the number of hit dice, 1d10 could be used to determine hit points, rather than 1d8. Perhaps this is why so many demons and devils are of this sort, something of a throwback to Eldritch Wizardry.

[Matthew]

Thinking about it some more, there seem to be two methods of advancement in AD&D, the "+1" hit point approach, and the +1 hit die approach. So, for instance, a weak goblin might have 1 hit point and stronger versions an additional point extra until plateauing at 7 hit points, but their saving throws and fighting ability remain the same throughout. A goblin of greater ability than one of maximum hit points is equivalent to an orc (HD 1), and then a hobgoblin (HD 1+1), but that seems to be where that approach "tops out", as more powerful "leader" types go up by full hit dice. This may help to explain why there is a separate entry on the combat matrices for "1−1", "1", and "1+1", but subsequent entries group multiple hit dice together:

LE = Life Energy; FA = Fighting Ability (fighter level equivalent); ST = Saving Throw (level equivalent)

HD 1: LE 1; FA 2; ST 1;
HD 2: LE 2; FA 5; ST 2;
HD 3: LE 3; FA 5; ST 3;
HD 4: LE 4; FA 6; ST 4;
HD 5: LE 5; FA 6; ST 5;
HD 6: LE 6; FA 8; ST 6;
HD 7: LE 7; FA 8; ST 7;
HD 8: LE 8; FA 9; ST 8;
HD 9: LE 9; FA 9; ST 9;
HD 10: LE 10; FA 11; ST 10;
HD 11: LE 11; FA 11; ST 11;
HD 12: LE 12; FA 12; ST 12:
HD 13: LE 13; FA 12; ST 13;
HD 14: LE 14; FA 13; ST 14;
HD 15: LE 15; FA 13; ST 15;
HD 16: LE 16; FA 14; ST 16;

Note: Saving Throw levels are effectively half rounded up, since most use the fighter progression which only advances at odd levels (much like the non-optional fighting ability of the same class).

Using the "pluses" the table looks more like this:

HD 1+1: LE 2; FA 3; ST 2;
HD 2+1: LE 3; FA 5; ST 3;
HD 3+1: LE 4; FA 5; ST 4;
HD 4+1: LE 5; FA 6; ST 5;
HD 5+1: LE 6; FA 6; ST 6;
HD 6+1: LE 7; FA 8; ST 7;
HD 7+1: LE 8; FA 8; ST 8;
HD 8+1: LE 9; FA 9; ST 9;
HD 9+1: LE 10; FA 9; ST 10;
HD 10+1: LE 11; FA 11; ST 11;
HD 11+1: LE 12; FA 11; ST 12;
HD 12+1: LE 13; FA 12; ST 13:
HD 13+1: LE 14; FA 12; ST 14;
HD 14+1: LE 15; FA 13; ST 15;
HD 15+1: LE 16; FA 13; ST 16;
HD 16+1: LE 17; FA 14; ST 17;

Note: Only HD 1 has a change in fighting ability.

HD 1+4: LE 2; FA 5; ST 2;
HD 2+4: LE 3; FA 5; ST 3;
HD 3+4: LE 4; FA 6; ST 4;
HD 4+4: LE 5; FA 6; ST 5;
HD 5+4: LE 6; FA 8; ST 6;
HD 6+4: LE 7; FA 8; ST 7;
HD 7+4: LE 8; FA 9; ST 8;
HD 8+4: LE 9; FA 9; ST 9;
HD 9+4: LE 10; FA 11; ST 10;
HD 10+4: LE 11; FA 11; ST 11;
HD 11+4: LE 12; FA 12; ST 12;
HD 12+4: LE 13; FA 12; ST 13:
HD 13+4: LE 14; FA 13; ST 14;
HD 14+4: LE 15; FA 13; ST 15;
HD 15+4: LE 16; FA 14; ST 16;
HD 16+4: LE 17; FA 14; ST 17;

Note: Every odd hit die moves up a class of fighting ability

Above "+4" is speculative for life energy levels. The sleep spell suggests that HD 4+4 is a significant cut-off point in that respect, but it is by no means certain; three-quarter, half, and quarter hit dice can probably be expressed as follows:

HD 1−3 (¼): LE 1; FA 0; ST 0;
HD 1−2 (½): LE 1; FA 0; ST 0;
HD 1−1 (¾): LE 1; FA 1; ST 0;

[Bargle]

Very interesting. A foot note to your berserkers. In chainmail, viking berserkers should receive, "+2 on their attack dice." So, this rule predates 0d&d hit dice.