Proficiency Bonus—the Player's Handbook explains twice how it works (p. 12 & 173–4). Here's the text from one of those places. #DnD pic.twitter.com/3mIpm5nQS0

— Jeremy Crawford (@JeremyECrawford) April 17, 2017

@DeanMSimmonsWhy not use existing word “Expertise” rather than new language “add twice Proficiency”? Clarity is good; stops munchkins in advance. The Expertise class feature itself doesn't use the word "expertise," which has no game meaning. It, too, talks about doubling.

— Jeremy Crawford (@JeremyECrawford) April 19, 2017