Non-Proficiency

Characters can use items with which they have no proficiency, but the consequences of doing so are usually too great to try.

Armor Non-Proficiency

A character who is wearing armor with which he is not proficient applies either a -4 penalty or the armor's armor check penalty (whichever is worse) to all attack rolls and to all skill checks that use STR or DEX.

Shield Non-Proficiency

When you are using a shield with which you are not proficient, you take either a -4 penalty or the shield's armor check penalty (whichever is greater) on attack rolls and on all skill checks that use STR or DEX.

Weapon Non-Proficiency

When using a weapon with which you are not proficient, you take a -4 penalty on attack rolls. Note that this is the same penalty as wielding an improvised weapon (i.e., it doesn't stack with the improvised weapon penalty).

All non-proficiency penalties stack with each other, so if you are wielding a weapon and a shield with which you are non-proficient, as well as wearing armor with which you are non-proficient, you would take at least a -12 to attack rolls, in addition to a minimum armor check penalty of -8 to all STR- and DEX-based skills.