Expertise is an attribute that indicates how skilled an adventurer is in using weapons or armor. Expertise is expressed as a percentage, with a low percentage representing unfamiliarity with the weapon or armor, and a high percentage indicating strong proficiency with it.
Weapons expertise is one factor used in combat to determine if an adventurer's attack is successful. (Others include the adventurer's agility and the quality or "complexity" of the weapon.) Every adventurer has an expertise rating for each of the five weapons classes, and starts with 5% for axes, -10% for bows, 20% for clubs, 10% for spears, and 0% for swords. Donald Brown notes in the Player's Manual that these initial values "reflect the fact that somebody who doesn't know what he's doing is more likely to hit with a club than with an arrow."
An adventurer's armor expertise value represents his ability to operate while wearing armor, and for new adventurers begins at zero.