Expertise

Expertise is now a defined general rule (it was previously a Rogue/Bard class feature). When a creature has Expertise in a skill or tool, it doubles its Proficiency Bonus for any ability check it makes using that skill or tool.

Mechanics:

• You must already be proficient in the skill or tool to gain Expertise in it. You cannot have Expertise in something you are not proficient with.
• The doubling applies only to ability checks that use the skill or tool — not to attack rolls, not to saving throws, not to the proficiency bonus itself in other contexts
• Multiple sources of Expertise in the same skill do not stack — the bonus is doubled once

Worked example: A 5th-level character (PB +3) with Expertise in Stealth adds +6 from proficiency instead of +3 to Dexterity (Stealth) checks. Their Dexterity modifier is added on top as usual.

Where Expertise comes from:

• Rogue class features (Expertise at 1st level, Reliable Talent and additional Expertise at higher levels)
• Bard class features (Expertise at 3rd and 10th level)
• Skill Expert feat (grants Expertise in one skill)
• Some backgrounds and species in expanded sources
• Magic items (gauntlets of ogre power do not grant Expertise; specific items like robe of the archmagi do not either — Expertise is rarely item-granted)