On a crit you add a dam dice then roll them twice?

@ChrisPerkinsDnD question.on a crit you add a dam dice then roll them twice? It's a little vague in the PHB. Thanks.

— Matt (@kreilickm) July 1, 2017

"All dice, roll twice; leave the bonus alone-us." For example, 2d6 + 3 damage becomes 4d6 + 3 damage on a crit. #WOTCstaff https://t.co/KYCRCiEAWF

— Christopher Perkins (@ChrisPerkinsDnD) July 1, 2017