G.11.1- Spells and abilities which prevent damage must be played (and resolve) before the damage is dealt, which happens when the spell or ability dealing the damage resolves. They set up a protective "shield" around the player or creature and prevent the damage from being dealt. [CompRules 1999/04/23]
G.11.2- Damage prevention spells and abilities can be played even if there is no damage to prevent at the time. [D'Angelo 1999/05/01]
G.11.3- If the spell or ability prevents a specific amount of damage, you choose which of the points of damage you want to prevent at the time the damage would be dealt. [D'Angelo 1999/05/01]
G.11.4- If the spell or ability restricts your choice, such as to just damage from a single source, you must follow that restriction. [D'Angelo 1999/05/01]
G.11.5- If the spell or ability has restrictions on the damage's source, such as only from a creature of a given color, those restrictions are checked when the damage would be prevented (in addition to when choosing the source). This means that if the source changes color (or whatever characteristic is being checked) that the damage might not be prevented. [WotC Rules Team 2000/04/04]
G.11.6- Unpreventable damage will not use up points of a damage prevention "shield" that counts points of damage. [WotC Rules Team 2002/02/01]
G.11.7- Unpreventable damage will not use up a general damage prevention "shield". [WotC Rules Team 2002/02/01]
G.11.Ruling.1- If damage is prevented, it is never considered "dealt", so any side-effects of that damage will also be prevented. [D'Angelo 1998/02/03]
G.11.Ruling.2- Damage prevention spells and abilities do not target damage. They just target what the card says they target. [WotC Rules Team 1997/07/03]
G.11.Ruling.3- Damage prevention spells and abilities often have you pick a source of damage. A source is a permanent, spell on the stack, or any card or permanent referenced by a spell, ability, or combat damage on the stack (including the source of that ability or damage if the source is no longer in play). You cannot choose other things, such as an unused Hammer of Bogardan in a player's graveyard. [CompRules 1999/11/01]
G.11.Ruling.4- It is possible to receive damage from the same source in multiple "chunks" at one time (as with Earthquake being used while Treacherous Link is in play, which would cause that creature's Earthquake damage to be redirected to the player). In this case, a single damage prevention shield (like a Circle of Protection: Red) can be applied to the merging of the "chunks" because all of the damage is being dealt at one time. [Barclay 2002/04/22]
Note- Also see Damage, Rule G.10.
Note- Also see Damage Redirection, Rule G.12.
Note- Also see Replacement and Prevention Effects, Rule T.10.