Skills with different abilities: How to adjudicate what combination to use?

Question

In the D&D 5e Player’s Handbook, there is a variant option called Skills with Different Abilities:

Normally, your proficiency in a skill applies only to a specific kind of ability check. Proficiency in Athletics, for example, usually applies to Strength checks. In some situations, though, your proficiency might reasonably apply to a different kind of check. In such cases, the DM might ask for a check using an unusual combination of ability and skill, or you might ask your DM if you can apply a proficiency to a different check.

Essentially, the rules allow for a DM to ask for an ability check but the player can apply a skill, normally associated with a different ability, to the check.

For example, attempting to blend into a crowd by engaging in conversation and acting like you’re meant to be there might call for a Charisma (Stealth) check. So the player would roll a Charisma check and apply their Stealth proficiency - if they have one - to the check.

However, not everything is clear cut, there are often overlapping skills which could equally apply. To use the same example, one could argue that it is not a Charisma (Stealth) check, instead it is a Charisma (Deception) as you’re deceiving the people about your intentions or (Persuasion) as you’re persuading them you’re meant to be there.

Another example, a player wants to parkour their way up to the top of some buildings. One could equally argue that this a Strength (Athletics) check as that is the usual ability associated with climbing or that it is a Dexterity (Athletics) check as you have to be exceptionally precise to quickly grab small ledges so you don’t fall to the ground.

So, my question is, when a player describes an action they want to take, how do you adjudicate which skill and ability to combine to make the check?

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Answers must be backed up with your experiences of using this variant rule

Answer 1

Ability first - then proficiency

The chapter is in the PHB is entitled Using Ability Checks - not Using Proficiencies.

Which ability applies depends on the 'approach' of the PC to the problem: that is, exactly what is the PC doing. With that firmly in mind, look at what each ability is used for and which of the six is most applicable to the task at hand:

A Strength check can model any attempt to lift, push, pull, or break something, to force your body through a space, or to otherwise apply brute force to a situation.

Dexterity measures agility, reflexes, and balance.

Constitution measures health, stamina, and vital force.

Intelligence measures mental acuity, accuracy of recall, and the ability to reason.

Wisdom reflects how attuned you are to the world around you and represents perceptiveness and intuition.

Charisma measures your ability to interact effectively with others.

If its an edge case I'll usually let the PC choose - after all they are making the approach so they can tailor it to their particular capabilities.

After deciding that - I throw it to the player to call up any skill or tool proficiency that they can persuade me applies. I don't set too high a bar for this - again they can play to their talents.

For your first example of blending into a crowd: Charisma (Stealth) - yes, Charisma (Deception) - yes, Charisma (Persuasion) - no, it's a personal foible but if you are telling outright lies you are deceiving, not persuading.

For your second example of parkouring up a building: Strength (Athletics) - yes, Dexterity (Athletics) - yes. Also Strength (Acrobatics) - yes, Dexterity (Acrobatics), yes.