This material is Open Game Content, and is licensed for public use under the terms of the Open Game License v1.0a.
THE BASICS

DICE NOTATION
These rules use the following die notations:
d4 = four sided die
d6 = six sided die
d8 = eight sided die
d10 = ten sided die
d12 = twelve sided die
d20 = twenty sided die
d% = percentile dice (a number between 1 and 100 is generated by rolling two different ten-sided dice. One (designated before rolling) is the tens digit. The other is the ones digit. Two 0s represent 100.)

Die rolls are expressed in the format:
[#] die type [+/- modifiers]
Example: 3d6+2 means: "Roll 3 six sided dice. Add the result of the three dice together. Add 2."

ROUNDING FRACTIONS
In general, if you wind up with a fraction, round down, even if the fraction is one-half or larger.
Exception: Certain rolls, such as damage and hit points, have a minimum of 1.

MULTIPLYING
Sometimes a special rule makes you multiply a number or a die roll. As long as you’re applying a single multiplier, multiply the number normally. When two or more multipliers apply, however, combine them into a single multiple, with each extra multiple adding 1 less than its value to the first multiple. Thus, a double (x2) and a double (x2) applied to the same number results in a triple (x3, because 2 + 1 = 3).

BASIC TASK RESOLUTION SYSTEM
These rules assume a standardized system for determining the success or failure of any given task. That system is:
d20 + Modifiers vs. Target Number
The Modifiers and Target Number are determined by the type of task.
If the result of the d20 roll + the Modifiers equals or exceeds the Target Number, the test is successful. Any other result is a failure.
A "natural 20" on the die roll is not an automatic success. A "natural 1" on the die roll is not an automatic failure, unless the rules state otherwise.