# Can I choose a temporary proficiency for Expertise?

I am a multiclassed bard 2/cleric 2 with the Knowledge Domain.

I use my Knowledge of the Ages Channel Divinity moments before leveling up and within the 10 minutes reach level 5 and choose to advance as a bard.

What happens if I select the proficiency granted by Knowledge of the Ages for Expertise? Because of Jack of all Trades would I double the 0.5d proficiency in that skill (and double my full proficiency if I gain proficiency on that skill later)?

I ask this question not in an attempt to power game but to fit a character archetype I'm designing (a lore master of sorts). While I know this concept is probably not optimal, it may be useful to get Expertise at this early stage on a skill that I don't necessarily have proficiency on now, but plan to have later. Additionally, I'm curious how this resolves in general.

• Comments are not for extended discussion; this conversation has been moved to chat. Apr 8, 2018 at 2:33

# You can probably do this, but it wouldn't work as you suggest

Channel Divinity: Knowledge of the Ages ...As an action, you choose one skill or tool. For 10 minutes, you have proficiency with the chosen skill or tool.

### Yes you can choose a temporarily proficient skill as long as your DM works with you

Seeing that Knowledge of the Ages makes you proficient in a skill temporarily, it seems that there is nothing preventing it from being a valid target for Expertise. It is probably not an intended use of it, but nothing in the rules prevents it.

However, it is going to take a carefully or luckily timed use of the Knowledge of the Ages ability coinciding with the moment of level-up for this to even get off the ground. This is going to probably involve some level of meta-gaming on the part of the player (and cooperation from the DM) (or some luck) to work. Whether this is frowned upon varies from table to table though and is certainly not inherently wrong.

So, as long as the DM is fine with this, there is no rules reason why you could not choose the Knowledge of Ages skill for the target of Expertise.

### If you are not proficient in a skill you gain no benefit from Expertise

Expertise does not grant proficiency, it only doubles the bonus of skill that you are already proficient in, when you make an ability check with it.

Expertise [C]hoose two of your skill proficiencies. Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.

Notice that Expertise applies only to proficiencies not skills in general. If the skill you are trying to apply it to isn't a proficiency for your character then Expertise cannot apply and nothing is doubled.

This is backed up by the PHB:

[I]f a feature or effect allows you to multiply your proficiency bonus when making an ability check that wouldn’t normally benefit from your proficiency bonus, you still don’t add the bonus to the check. For that check your proficiency bonus is 0, given the fact that multiplying 0 by any number is still 0. For instance, if you lack proficiency in the History skill, you gain no benefit from a feature that lets you double your proficiency bonus when you make Intelligence (History) checks. (PHB 174)

Thus, if you are not proficient in a skill, that skill can gain no benefit from Expertise at all. Even if you are getting a bonus from Jack of All Trades, that skill is still not a proficiency for you and thus cannot be a target of Expertise in the first place.

This is how is plays out in each case:

### While Knowledge of Ages is active

While you are still proficient in the skill you would add your proficiency bonus to it. Thus Jack of All Trades would not apply to it. Thus it would be $$AbilityMod + ProfBonus*2$$

### When Knowledge of Ages runs out

As soon as Knowledge of the Ages ran out, you completely lose proficiency in that skill. Since that skill no longer counts as one of your skill proficiencies you gain no benefit from Expertise (per the PHB). Thus, only Jack of All Trades applies and add half of your proficiency bonus to the ability check.

The calculation would now be $$AbilityMod + ProfBonus*0.5$$

### Knowledge of Ages gets activated again

If you gain proficiency in that skill again (permanently or temporarily) then the skill is now a valid target for Expertise again. In this case, the calculation would go back to $$AbilityMod + ProfBonus*2$$

• Any analysis on how the non-proficiency expertise would impact Jack of all Trades? Apr 7, 2018 at 20:25
• @SeraphsWrath: Because you choose it as the permanent target of Expertise. The ability only applies when the target is valid. The skill is only a valid target when they have proficiency in it. Apr 7, 2018 at 20:48
• @SeraphsWrath it's really rather simple. Imagine proficiency as a computer, and expertise as a high-end graphics card. Now, if you have the computer, you can build in the graphics card and enjoy both. However, without the computer, the graphics card is useless - but you still have it and can build it back in when you get the computer back. (Additionally, the situation in this question would only allow you to buy a graphics card when you currently own a computer - but you still have the card if the latter changes) Apr 8, 2018 at 2:30
• I read it as Expertise requires you to choose a proficiency, but doesn't state that the calculation requires you to be proficient, only that it applies to the chosen proficiency. Any check that uses that proficiency is doubled, there is no mention of knowledge of that proficiency being required during the check itself. Apr 8, 2018 at 9:09
• @Rubiksmoose I disagree, it is a proficiency in terms of the game mechanics, and if a check uses that proficiency from a mechanical point of view then you arguably benefit from expertise, having chosen it at a point in time when you did have the proficiency - which is all that the RAW description requires. Apr 8, 2018 at 13:16

# Yes

This specific case is addressed on pages 173-174 of the PHB.

If a feature or effect allows you to multiply your proficiency bonus when making an ability check that wouldn’t normally benefit from your proficiency bonus, you still don’t add the bonus to the check. For that check your proficiency bonus is 0, given the fact that multiplying 0 by any number is still 0. For instance, if you lack proficiency in the History skill, you gain no benefit from a feature that lets you double your proficiency bonus when you make Intelligence (History) checks.

So, yes, you are expected to be able to end up with Expertise in things you don't have proficiency in somehow, despite the fact the system also tries to hinder your ability to do that. If you do achieve Expertise with a skill you lack proficiency in, your proficiency modifier for that skill is 0. With Jack of All Trades, however, your proficiency modifier would end up at its usual total after all the multiplying and dividing.

Note, however, that by doing this you are locking yourself into having expertise in that skill, which you would probably benefit more from tying to a skill you use frequently and have proficiency in. You don't get a whole lot of expertise choices, after all.

## You technically still have Expertise in the Skill

Expertise allows you to:

Choose two of your skill proficiencies. Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.

Once Knowledge of Ages expires, you will still have Expertise in the skill, but you won't have any Proficiency Bonus to double for that skill. Keep a note about having Expertise for that particular skill around, because if you reacquire the skill, you will still have Expertise in it.

For every skill you are Proficient in, you get to add your Proficiency Bonus to checks involving that skill. Expertise allows you to double that. If you don't have Proficiency in a Skill, you add nothing to the skill's use, and doubling nothing from Expertise still gets nothing. $$2 * 0 = 0$$

This is backed-up in the PHB on pages 173-174:

If a feature or effect allows you to multiply your proficiency bonus when making an ability check that wouldn’t normally benefit from your proficiency bonus, you still don’t add the bonus to the check. For that check your proficiency bonus is 0, given the fact that multiplying 0 by any number is still 0. For instance, if you lack proficiency in the History skill, you gain no benefit from a feature that lets you double your proficiency bonus when you make Intelligence (History) checks.

So, you still have the Expertise, which still doubles whatever you do add from Proficiency, but just having Expertise does not give you the ability to add your Proficiency Modifier. However, if you get any Proficiency modifier through another trait, then the text implies that you do double that modifier, when it says,

For that check your proficiency bonus is 0, given the fact that multiplying 0 by any number is still 0.

Now, for how this works with Jack of All Trades

Starting at 2nd level, you can add half your proficiency bonus, rounded down, to any ability check you make that doesn't already include your proficiency bonus.

With the way this works, if you chose to do this, you would be adding the full Proficiency Modifier to the Ability Check, but only for that specific Skill, because adding two halves gets a whole value. $$2 * 0.5 = 1$$

• Expertise only doubles the proficiency bonus on ability checks using a skill proficiency. How are you applying it to a skill that isn't a proficiency for that character? Apr 7, 2018 at 21:17
• @Rubiksmoose because using a particular proficiency is a property of the skill check (see page 174 of the PHB which clearly and explicitly lays out the exact same argumentation provided in this answer) Apr 7, 2018 at 21:26
• @Rubiksmoose Re-write was long enough I just posted my own answer with it. SeraphsWrath can figure out how it fit it in their answer themselves, if they want ^^; Apr 7, 2018 at 21:35