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Lets say there exist some enterprising young bard 3/ sorcerer 1 named Kirk.

Deciding to uphold (long past) family tradition, he has chosen to go out and have a little half-dragon of his own.

He chose Persuasion as his expertise skill, and is of a dragon bloodline himself, gaining the Dragon Ancestry perk.

Do they stack? Not the same ability, and both say "Your proficiency is doubled" when they activate. Granted, it would only happen when dealing with dragons. But there are certainly worse times to ramp up your charm.

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marked as duplicate by Rubiksmoose dnd-5e Nov 27 '18 at 14:56

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Do they stack?

RAW, no.

Occasionally, your proficiency bonus might be multiplied or divided (doubled or halved, for example) before you apply it. For example, the rogue's Expertise feature doubles the proficiency bonus for certain ability checks. If a circumstance suggests that your proficiency bonus applies more than once to the same same roll, you still add it only once and multiply or divide it only once. (PHB pp.173-4, emphases mine)

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Your proficiency is doubled only once

On PHB p. 173:

Your proficiency bonus can’t be added to a single die roll or other number more than once. For example, if two different rules say you can add your proficiency bonus to a Wisdom saving throw, you nevertheless add the bonus only once when you make the save.

Occasionally, your proficiency bonus might be multiplied or divided (doubled or halved, for example) before you apply it. For example, the rogue’s Expertise feature doubles the proficiency bonus for certain ability checks. If a circumstance suggests that your proficiency bonus applies more than once to the same roll, you still add it only once and multiply or divide it only once.

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In general, things don't stack because the PHB specifically indicates the starting point:

Dragon Ancestor (PHB p.102, emphasis mine):

At 1st level, you choose one type of dragon as your ancestor. The damage type associated with each dragon is used by features you gain later.

You can speak, read, and write Draconic. Additionally, whenever you make a charisma check when interacting with dragons, your proficiency bonus is doubled if it applies to the check.

Expertise (PHB p.54, emphasis mine):

At 3rd level, choose two of your skill proficiencies. Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.

At 10th level, you can choose another two skill proficiencies to gain this benefit.

Neither of these modify your proficiency bonus, only what is done to the roll.
This means:

  • dragonAncestorBonus = proficencyBonus × 2
  • expertiseBonus = proficencyBonus × 2

Since both start with your proficiency bonus, you get to choose:

  • total = roll + proficencyBonus
  • total = roll + dragonAncestorBonus
  • total = roll + expertiseBonus

To help make things more clear, the PHB states (p173-174):

Your proficiency bonus can't be added to a single die roll or other number more than once. For example, if two differenct rules say you can add your proficiency bonus to a wisdom saving throw, you nevertheless add the bonus only once when you make the save.

Occasionally, your proficiency bonus might be multiplied or divided (double or halved, for example) before you apply it. For example, the rogue's Expertise feature doubles the proficiency bonus for certain ability checks. If a circumstance suggests that your proficiency bonus applies more than once to the same roll, you will add it only once and multiply or divide it only once.

To get around this constraint, the ability would need to be similar to "increases Persuasion proficiency by 5" or "adds 4 to all persuasion checks."

For the most part, you will not find major stacking effects in D&D-5e. If you think you found something which is broken or too good to be true, it probably is.

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