Dice Rules

Game Dice
In these rules, the different dice are referred to by the
letter d followed by the number of sides: d4, d6, d8, d 10,
d 12, and d20. For instance, a d6 is a six-sided die (the
typical cube that many games use).
Percentile dice, or d 100, work a little differently. You
generate a number between 1 and 100 by rolling two
different ten-sided dice numbered from 0 to 9. One die
(designated before you roll) gives the tens digit, and
the other gives the ones digit. If you roll a 7 and a 1, for
example, the number rolled is 71. Two 0s represent 100.
Some ten-sided dice are numbered in tens (00, 10, 20,
and so on), making it easier to distinguish the tens digit
from the ones digit. In this case, a roll of 70 and 1 is 71,
and 00 and 0 is 100.
When you need to roll dice, the rules tell you how
many dice to roll o f a certain type, as well as what modifiers
to add. For example, “3d8 + 5” means you roll
three eight-sided dice, add them together, and add 5
to the total.
The same d notation appears in the expressions “1d3”
and “1d2.” To simulate the roll of 1d3, roll a d6 and
divide the number rolled by 2 (round up). To simulate
the roll of 1d2, roll any die and assign a 1 or 2 to the roll
depending on whether it was odd or even. (Alternatively,
if the number rolled is more than half the number of
sides on the die, it’s a 2.)

The D20
Does an adventurer’s sword swing hurt a dragon or just
bounce off its iron-hard scales? Will the ogre believe an
outrageous bluff? Can a character swim across a raging
river? Can a character avoid the main blast o f a fireball,
or does he or she take full damage from the blaze? In
cases where the outcome of an action is uncertain,
the Tir Na Nog’s game relies on rolls o f a
20-sided die, a d20, to determine success or failure.
Every character and monster in the game has capabilities
defined by six ability scores. The abilities are
Strength, Dexterity, Constitution, Intelligence, Wisdom,
and Charisma, and they typically range from 3 to 18
for most adventurers. (Monsters might have scores as
low as 1 or as high as 30.) These ability scores, and the
ability modifiers derived from them, are the basis for
almost every d20 roll that a player makes on a character’s
or monster’s behalf.
Ability checks, attack rolls, and saving throws are the
three main kinds of d20 rolls, forming the core of the
rules of the game. All three follow these simple steps.
1. Roll the die and add a modifier. Roll a d20 and
add the relevant modifier. This is typically the modifier
derived from one of the six ability scores, and it
sometimes includes a proficiency bonus to reflect a character’s
particular skill. (See chapter 1 for details on each
ability and how to determine ability’s modifier.)
2. Apply circumstantial bonuses and penalties. A
class feature, a spell, a particular circumstance, or some
other effect might give a bonus or penalty to the check.
3. Compare the total to a target number. If the total
equals or exceeds the target number, the ability check,
attack roll, or saving throw is a success. Otherwise, it’s
a failure. The DM is usually the one who determines
target numbers and tells players whether their ability
checks, attack rolls, and saving throws succeed or fail.
The target number for an ability check or a saving
throw is called a Difficulty Class (DC). The target
number for an attack roll is called an Armor Class (AC).
This simple rule governs the resolution of most tasks
in Tir Na Nog play. Chapter 7 provides more detailed rules for
using the d20 in the game.