Melee mastery boosts melee weapon skill, same for missile mastery and ranged weapons. The general idea is that if you have 100 melee mastery and 50 small edged, you could hint something at about 75 ranks worth of skill (note this is an example, the bonus provided by melee mastery is not this straight-forward).

Melee mastery can't boost your highest weapons of that type beyond their effective skill IIRC.

Melee and Missile Mastery skills provide a percent bonus to weapons based on Kodius previously commenting that they can provide more than the 20% bonus cap that abilities are tied to. Both skills do indeed bonus even your highest weapon skill. You can actually gauge your ability to hunt a critter, offensively speaking, by your mastery skills especially if they are higher than your weapon skills (which isn't hard to do).

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