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Master is the fifth (and currently final) stage of a profession learned at the appropriate (see below) profession trainer.

Additional notes:

• Not counting any racial or other bonuses, at a certain skill level — 300 skill — you can learn the Master stage and gain up to a maximum of 375 points.
• You must be level 50 to become a Master. (Gathering professions appear to have no minimum level)
• The trainer who will train Master stage for each profession is usually called a master trainer and can only be found in an area accessible to owners of the Burning Crusade expansion.
• No skill or profession can go above 375 points (not including racial or other bonuses).

Profession skill levels fall into five stages of increasing ability to learn and execute recipes: Apprentice, Journeyman, Expert, Artisan, and Master (-only).