Helpful Game Mechanics Info

(Short for “To Hit Armour Class 0”)

For all Classes your Base THAC0 score is dictated by your Class As Follows:

Group/Level 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Warrior 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
Priest 20 20 20 18 18 18 16 16 16 14 14 14 12 12 12 10 10 10 8 8
Rogue 20 20 19 19 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 11
Wizard 20 20 20 19 19 19 18 18 18 17 17 17 16 16 16 15 15 15 14 14

This is then further modified DOWN (Despite them being expressed as a “+”) for Melee attacks by:
Specialisation Bonus (+1)
The ‘To Hit’ adjustment from your Strength (Varies)
Magical bonuses of the Weapon (Varies)
Nonmagical (Special Material) bonuses of the weapon (Varies)

Or further modified DOWN (Despite them being expressed as a “+”) for Missile attacks by:
Specialisaton Bonus to Hit at Point Blank (30’) with Bows and Crossbows) (+1)
The ‘Missile’ adjustment from your Dexterity (Varies)
Magical bonuses of the Weapon (Varies)
Nonmagical (Special Material) bonuses of the weapon (Varies)

For all attacks/Weapons you can use, it is useful to know your “Modified” THAC0 score. You’ll see why, next.

THAC0 Roll
Once your THAC0 score is known, you’re ready to roll To Hit. To do so, roll 1d20. A roll of 1 is a Fumble (bad things happen – usually causing you to miss your next attack at least). A Roll of 20 is a Critical (Good things happen – usually Roll Damage Dice for weapon, and Double it, before applying modifiers).

Subtract the Dice roll from your modified THAC0 score. The number you have remaining is the Armour Class you hit.

Example: JJ Has a THAC0 of 18 with his Longsword. He rolls to hit and scores a 11. 18 (THAC0) – 11 (Roll) = AC 7.
Example: A little later in the fight, he rolls a 19. 18 (THAC0) – 19 (Roll) = AC -1.


Each PC rolls 1d10, and subtracts the Reaction Adjustment modifier from their Dexterity. They then add the Casting Time, or Weapon Speed of whatever they are doing. Unarmed attacks have a ‘Speed’ based on your Size Category. Size S/M Creatures are Speed 3 in Unarmed Combat.

Example: JJ Rolls 5 on his Initiative Dice, and plans to attack with his Quarterstaff (which, thanks to 2-Hander Style, is Speed 1). His Dexterity is 19, so his Reaction Adjustment is +4. This gives him an Initiative of (5 – 4) +1 = 2. Should he choose to attack with another weapon, in a later round, his Initiative Score is modified accordingly.
Example: JJ Chooses to hit with his Longsword in round 3. The Longsword is a Speed 5 weapon, which means he changes his Initiative Score to (5 – 4) +5 = 6.

What Needs High Rolls, and What Needs Low Rolls

THAC0 (To Hit) Rolls.
Saving Throws.
Turning Undead.
Damage Rolls.

Stat Rolls (such as Dex Checks, or Open Doors rolls).
Proficiency Checks.
Rogue-Skill Uses.
Learn Spell Checks (Being ABLE to understand a NEW Spell, not Morning Memorisations).
Encounter/NPC Reaction Checks

Additional Notes:
TSR Never got the hang of Bonus and Penalty notations. Usually Bonuses are written as a “+”, even if they are SUBTRACTED from the Number; +3 to Hit actually LOWERS your THACO by 3.
However. They cannot even stay consistent in that. – 4 Defensive Bonus (a Dexterity Trait) actually Lowers your AC by 4 points. Despite being written as a “-”.
Usually, its best to tell what you are supposed to do, based upon whether it says it is a “bonus” or a “penalty”, rather than the mathematical symbol. If it is a Bonus, it moves the score in whichever way most benefits the Character. If it is a Penalty, it moves it in the opposite direction.


Most proficiencies have a score. There are a couple of exceptions, but not many. For those exceptions, it’s a case of “you can or you can’t”. If you have it, you can. If you don’t, you can’t. The majority of Profieincy Scores are calculated based upon your raw Stat, with a modifier. Such as “Juggling, Dex -1”. Assuming a Dex of 15, this would give you a Juggling Proficiency of 14 (Dex -1). Occasionally the modifier will be a Bonus (Riding, Land Based (Horse) Wis +3), but normally it will be 0, or a penalty.

Without a shadow of a doubt, one of the most difficult proficiencies to have at high levels is Tracking. It applies a staggering -6 penalty to the characters Wisdom Score! Rangers have an advantage, in that they do NOT receive the -6 penalty. Some Ranger Kits even give bonuses to their Tracking skills. This is why Tracking is almost always the sole reserve of the Ranger.

Helpful Game Mechanics Info

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