Core rule · Rules Tome 2.0.8 § 105.2
Advantage and disadvantage
Advantage keeps the higher of two d20s; disadvantage keeps the lower.
How it works
Full rule
Multiple instances do not add more dice. Any amount of advantage and disadvantage cancel each other, producing one normal roll. Planar changes which eligible die is retained for affected strikes.
At the table
Example
Two sources of advantage still roll two dice. One source of disadvantage then cancels all of that advantage and only one die is rolled.
Watch for this
Common mistake
The player does not freely choose either die: advantage keeps the higher and disadvantage keeps the lower, subject to Planar’s special rule.