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Implement proficiency allows a character to apply the benefits of a wielded implement to implement keyword powers.

Implement proficiency can be gained as a class trait:

Characters with multiple classes can use any implement with which they are proficient with any of their implement keyword powers. For example, a character who is both a cleric and a wizard can use a holy symbol, a cleric's implement, when using wizard powers, and vice versa.

Originally, 4th edition characters had to use implement powers of a class with an implement usable by the same class. The Essentials line of rulebooks simplified the use of implements such that a character can use any implement with which he or she is proficient for any implement power.