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In the rules on "Combining Magical Effects" (PHB, p. 205), the bless spell is given as the example of spell effects not being combined (not 'stacking'):

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.

For example, if two clerics cast bless on the same target, that character gains the spell’s benefit only once; he or she doesn’t get to roll two bonus dice.

I'm with this all the way up to the final semi-colon.

I get that the target cannot benefit from two of the same spell at the same time; I don't understand why the target doesn't get to roll two bonus dice.

The description itself says "the most potent effect—such as the highest bonus—from those castings applies". In the specific case of bless, the bonus is variable and is determined in each instance referenced by rolling a d4. It is impossible to know which of the two effects is the more potent without rolling two dice.

The process described of taking the higher bonus but not combining the effects seems to me to dictate that a target under the effects of two bless spells would roll two bonus dice, would determine which one was higher, and then would be affected only by the higher one (not combine them). Such a procedure would fit the requirements that the spells "don't combine", that the target gains the spell's benefit only once, and that the most potent effect applies. To not roll two bonus dice would mean that the target was not necessarily receiving the more potent effect.

To me it seems that rolling dice is not gaining a benefit from the spell, using the bonus rolled is gaining a benefit, and that is done only once.

What am I misunderstanding?

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The magical effect of bless is:

Whenever a target makes an attack roll or a saving throw before the spell ends, the target can roll a d4 and add the number rolled to the attack roll or saving throw.

When determining “which effect is more potent”, this is what we are evaluating. To be precise, the result of the d4 roll is not the effect we are evaluating, the spell description is. It is unambiguous that determining which effect is more potent is done before the effects are applied: rolling the dice is applying the effect since the effect says roll a d4, and deciding afterwards requires rolling twice, something explicitly forbidden in the rule cited:

he or she doesn’t get to roll two bonus dice.

To put it another way, we can use a reductio ad absurdum argument to get the right approach.

  1. Suppose we determine which bless to apply after seeing which rolls the greater bonus.
  2. We roll two dice.
  3. This contradicts the rule cited “he or she doesn’t get to roll two bonus dice”.
  4. Therefore we determine which bless to apply before seeing which rolls the greater bonus.
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  • \$\begingroup\$ The example given of the 'most potent effect' is "the highest bonus". So you are saying that both bless spells give a bonus of "d4", rather than one gives a bonus of +3 and another +2 this round. Would you also then say that a single bless spell gives "the same bonus every round" (d4)? I can understand that as a resolution of my question, but it is not how I would use the word bonus. \$\endgroup\$ – Kirt Jul 14 '20 at 23:51
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    \$\begingroup\$ Yeah, that’s right. \$\endgroup\$ – Thomas Markov Jul 14 '20 at 23:53
  • \$\begingroup\$ It is not a “same bonus every round” spell in the sense that you roll once for the whole spell duration. Every time bless would apply, you roll 1d4, no matter how many times you are blessed. \$\endgroup\$ – Thomas Markov Jul 14 '20 at 23:55
  • \$\begingroup\$ "it is unambiguous that determining which effect is more potent is done before the effects are applied;" If two rolls were allowed, determining which bonus was more potent would be done before the effects were applied. The effects are applied when the resulting bonus modifies a die roll. Deciding between two die rolls which to apply is done before the results are applied, not after. \$\endgroup\$ – Kirt Jul 15 '20 at 0:00
  • \$\begingroup\$ Rolling the dice is applying the effect since the effect says roll a d4. \$\endgroup\$ – Thomas Markov Jul 15 '20 at 0:01
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Neither Bless is "more potent"

They both allow the affected person to add 1d4 to an attack roll or saving throw - this is the same potency. The player does not know ahead of time the result of either roll so there is no difference between the effect - add 1d4.

Therefore the DM decides.

For most practical purposes, it doesn't matter

If a character is under the effects of 2 (or more) Bless spells, then they can choose to use one and that one no longer affects them. They can then use another on a subsequent roll (and so on).

Bless has a duration of 1 minute which is plenty to cover any single combat so deciding which Bless was used and which wasn't has little practical effect apart from knowing which caster has to keeep concentarting and which doesn't.

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    \$\begingroup\$ It does not even matter for concentration. The spell that is "not in use" is still active. If the other spell ends, it will take effect. So either caster can stop concentrating and the result wil be the same. \$\endgroup\$ – Szega Jul 15 '20 at 1:52
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    \$\begingroup\$ The version of the "Combining Magical Effects" rule as quoted in the question leaves out a part of the rule that was added in errata: "Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap." Since multiple castings of bless are basically always equally "potent", this seems like the part that would be most applicable for bless. \$\endgroup\$ – V2Blast Jul 29 '20 at 23:26
  • \$\begingroup\$ ...That was regarding the second half of your answer. (Also, I noticed that "concentrating" is misspelled in the last sentence, so you may want to fix that if you keep that sentence in your answer.) \$\endgroup\$ – V2Blast Jul 29 '20 at 23:28
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The potency of the spell is not determined by the end dice roll

Potency is, as defined by Webster:

the ability or capacity to achieve or bring about a particular result

So to determine potency, you have to examine the capacity, not a singular end result.

For a quick example; consider Fireball. Cast at 3rd level, it does 8d6 damage.

  • If Waldo Wizard casts it at 3rd level and gets crappy dice rolls, it will only hit for 8 points of damage.
  • But Wario Wizard casts it at 3rd level and uses loaded dice to get 48 points.

Both have the same potency (capacity), but one did more damage. In DM description terms that could be caused by moving into or out of the effect, luck, ducking under cover, divine intervention, etc. It's the same spell, same capacity, but with different results.

Also keep in mind that results changes every time a spell is used. It keeps the same capacity/potency but the results can vary greatly.

So just because one Bless could boost +1 one time and +4 on another, it is still the same spell, cast once, with the same potency.

So how can someone measure potency?

First, go give this answer a read.

Although it's focused more on damaging spell rather than bonuses, we can use a similar scale.

Bless is a 1st level spell. Anyone who receives the spell cast at 1st level gets a d4 added to their attacks and saving throws. What if someone casts it at 2nd level? There is a section for that.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, you can target one additional creature for each slot level above 1st.

So the bonus does not change; only the number of targets. So for Bless, no matter who casts it, and at any level, the potency is still the same; 1d4. There is no higher, better, faster, stronger version of the bonus regardless of caster.

And since there is no "most potent", any one casting can be assumed. The effects overlap, but only one can be truly active at a time.

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    \$\begingroup\$ I didn't downvote but I think your answer could benefit by giving examples of what does determine the potency (greater damage potential, higher saving throw, etc...) \$\endgroup\$ – user60913 Jul 14 '20 at 23:36
  • \$\begingroup\$ The version of the "Combining Magical Effects" rule as quoted in the question leaves out a part of the rule that was added in errata: "Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap." Since multiple castings of bless are basically always equally "potent", this seems like the part that would be most applicable for bless. \$\endgroup\$ – V2Blast Jul 29 '20 at 23:28
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No explicit guidance is given as to how to tell what 'more potent' means

That means we go with what the words mean in 'normal' English. That's pretty clear here: you'd roll two bonus dice and use the higher number.

But wait! Actually, you explicitly do not do that! So, clearly something special is meant by 'most potent', but it's not defined anywhere.

At this juncture, many groups resort to various guesses at or specialized definitions about what 'most potent' might then mean. Some of the most common follow. Keep in mind that these are often used in conjunction, with one metric taking precedence and another coming in on ties, sometimes with many many layers:

  • spell slot level

  • caster level (i.e. the level of the character casting the spell)

  • quantity of bonus or penalty added

    • variable bonuses and penalties are either treated as their average roll, or rolled and then considered, or their maximum roll. Sometimes this breaks the bless example
  • quality of effect added (e.g. Aquatic Adaptation alter self is a bigger kind of deal than Change Appearance alter self, and so is 'more potent' so long as both retain their selected effects)

  • remaining duration

  • the one the player/PC picks

  • the one that the DM guesses is worst for the player/PC

In the end, though, we don't know. All we can really know is that if you want to play with the errata trying to keep spells from meaningfully stacking with themselves, you've got to do a bit of work finishing the rules about that before you can play with them. Worth noting is that, should you decide that the spells are equally potent, then more-recent errata provides that the most recent spell takes precedence. That's only if you decide two spells are equipotent, though, and that depends on what potency means, as discussed above.

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    \$\begingroup\$ The version of the "Combining Magical Effects" rule as quoted in the question leaves out a part of the rule that was added in errata: "Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap." If multiple castings of bless are basically always equally "potent", this seems like the part that would be most applicable for bless. \$\endgroup\$ – V2Blast Jul 30 '20 at 22:29
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    \$\begingroup\$ Oh, sure, but only once you've decided they're equipotent. \$\endgroup\$ – Please stop being evil Jul 30 '20 at 23:20

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