Advantage

Roll the d20 twice and use the higher result.

• Several sources don't stack — it's still two dice.
• If you have both Advantage and Disadvantage, they cancel out and you roll one die.

Other rules

• Concentration
• Disadvantage
• Armor Class
• Hit Points
• Temporary Hit Points
• Hit Point Dice
• Proficiency Bonus
• Saving Throw
• Spell Save DC
• Initiative
• Short Rest
• Long Rest
• Heroic Inspiration