Editing Unwritten: System Reference Document/Game Basics (section)

===Invoke an Aspect===

An aspect can be invoked by a player in appropriate situations to improve a character’s chances. For example, Hotshot Big Game Hunter could be invoked when when doing something really dangerous, because you are used to taking big risks.

Whenever you’re making a roll, and you’re in a situation where an aspect might be able to help you, you can invoke it in order to shift the dice result. This allows you to either reroll the dice or shift the roll by +2, whichever is more helpful. You do this after you’ve rolled the dice and you aren’t happy with your total. You also have to explain or justify how the aspect is helpful in order to get the bonus—sometimes it’ll be self-evident, and other times it might require some creative description.

You spend fate points to invoke an aspect. You can spend more than one fate point on a single roll, gaining another reroll or an additional +2, as long as each point you spend invokes a different aspect. You can also roll to gain ‘free invocations’, which allow you to invoke an aspect without a fate point.

When stopped at a gas station near the deserts above the D’ni caverns, Naomi discovers that someone has stolen a Linking Book from her car. She sees a figure run off behind the dunes and follows. When she rolls to see if she can track the thief, she spends a fate point to invoke her Desert Rat aspect, which gives her a +2 bonus.