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Stances are abilities that a Warrior can use in order to change the effects and availability of his attacks.
Stances are different positions a Warrior can take in order to enhance his offensive or defensive abilities. In game mechanics terms, stances change the way your Warrior performs. Stances can apply a global modifier to your threat output or damage intake, and will also limit or expand the list of available moves you can use.
Additionally, when changing a stance will reduce your Rage to a maximum point; by default this is zero, but once you train Stance Mastery at level 20 you can retain up to 10 Rage. Each point of Tactical Mastery also lets you retain up to five additional points of Rage when changing stances.
Battle Stance is your normal, balanced stance with no special penalties or bonuses to attack or defense. Some of the Warrior's most powerful and iconic abilities are located in this stance - Charge, Overpower and Mocking Blow. While in Battle Stance, you generate only 80% of your normal threat.
Defensive Stance is your typical tanking stance, with a 10% reduction in damage dealt and received as well as a 30% threat generation bonus. The vast majority of the moves you will use when tanking are located in this stance - Shield Block, Revenge and Disarm, plus several others.
Berserker Stance is your offensive stance, with a 10% increase in damage received and a 3% increase to your critical strike chance. Unlike Defensive Stance, as you may have noticed, the modifiers are not equal - you'll receive more damage, but the increase in output from the Stance alone won't alone compensate for the increased damage input. However, Berserker Stance has some of the Warrior's best offensive abilities - Berserker Rage and Whirlwind.