Proficiency

Proficiency is the game's word for training. It is a single bonus, the same size for everything you are trained in, and it grows with your character's level rather than with practice.

The bonus depends on your total character level and nothing else. It is the same number whether you are swinging a sword you were trained with or recalling a piece of lore you studied.

Where it applies

You add your proficiency bonus when you are trained in what you are doing, and not otherwise. In practice that means:

• a skill you are proficient in, on an ability check that uses it;
• a saving throw in one of the two abilities your class grants;
• an attack roll with a weapon you are trained in, or with your spells.

There is no half-credit. Either the bonus applies in full or it does not apply at all, which is what keeps the arithmetic on your sheet short.

You never add it twice to the same roll, either — with one exception.

Expertise

Expertise doubles your proficiency bonus, and only for the specific skills it names. At level 1 that is the difference between +2 and +4; at level 17 it is +6 against +12, which is wide enough that an expert character succeeds at things others cannot seriously attempt.

It applies to the skill, not the ability. Expertise in Stealth does nothing for a Dexterity check to catch a falling rope.