There is no single area of D&D 5e that provides more confusion than Ability (Skill) checks. On this page, I outline my interpretation of the RAW rules, and also the house rules and clarifications I use when adjudicating these checks. Originally, I just had a few, but realized I was developing a suite of rules, and thought it would be useful to consolidate and explain.
Ability checks, which some call "skill checks" are actually ability checks, meaning you are checking if you are strong enough, smart enough, wise enough, etc., to perform the task at hand. If you have training in a relevant skill, then you can add your proficiency bonus to the check, but at the core of it, these are ability checks, not skill checks. That's RAW. That's also why they are listed in official rules as an Ability (Skill) check, e.g., "Dexterity (Acrobatics)" not just "Acrobatics."
Characters are defined by their abilities, their experience, and their proficiencies. Some are very strong (Str 18 = +4 bonus to Str based checks), are very experienced (at level 9 your Proficiency bonus becomes +4), and trained in some skills (e.g., Athletics, a Str based skill). That dude would make a Strength (Athletics) check at +8. A similar dude without Athletics training would just have +4 to their Strength check.
DC, or Difficulty Class, is the measure of how difficult a skill check is. Generally, a 10 is easy, a 15 is medium, and a 20 is hard. DMs are free to decide on the DC of any check, though often the difficulty is laid out in printed adventures. Ties (in this case, matching the DC) go to the players.
Passive Ability scores are a number reflecting a baseline of skill. Passive Perception, for example, is 10 plus your ability modifier, plus your proficiency bonus if you are proficient in the skill at hand. Passive scores are relevant to all the ability/skill pairs, so for the dude above, his Passive Strength (Athletics) score is 18. Dude is pretty strong for athletics related tasks like climbing, jumping, or swimming. Given enough time, he could easily accomplish easy and medium athletic tasks with no problem.
When it comes to making an Ability (Skill) check, this is how I roll.
And a rule clarification for a very specific case...
Since these are Ability checks, not skill checks, it follows that you do not need to have any special proficiency to use your abilities. You're just not as good as someone with specific training. At first level, having proficiency adds +2 to an ability check, which translates to 10% more chance of success. By 9th level, that is up 20% (+4).
That said, there is an argument to be made that an acolyte having grown up around religious tomes should be more than 10% better at a Religion check than the barbarian who thinks religion involves throwing bones. The +2 bonus for proficiency doesn't seem like enough in some circumstances. Therefore, I am always open to ruling that proficiency is required in some cases. I would consider the situation, a character's background as well as their proficiencies, and whether just increasing the DC would be acceptable (rather than requiring a skill).
For example, a party is confronted by an ancient temple altar, and wish to investigate it for clues about the temple's inhabitants. Clearly someone proficient in Religion could use that to bolster their Intelligence check. But could someone without Religion even attempt to decipher the altar's meaning? I would say, sure! I mean, evil has a certain look, right? Blood on the altar, spiky letters, skulls embossed in the stonework? That's not so hard. And if it is hard, say just a plain altar with one symbol carved into it, maybe the DC is more difficult, like a 20 instead of a 15? But if the party is looking for something specific, like if the altar could be used to create a lich, I would rule that would be outside of the general knowledge area, and would require a Religion proficiency (or Arcana, arguably, to raise a lich).
This is RAW, but lots of players get it wrong, crowing or slumping when they roll a 1 or a 20 on their Wisdom (Perception) check. There is no critical failure or success on an Ability check. Period.
A high Passive score does not equal an automatic success. In my house rule, a passive score that beats the DC allows a character an instinctive advantage (essentially, the DM tells the player they can make an active check). Perception is the one that comes up most often. As I rule it, a high Passive Wisdom (Perception) sets off a character's senses, so that they know something is up, and can then take an action based on that, perhaps making an active check, or telling others to look around, etc.
If your Passive score is high enough to meet or beat the DC of a task, you can assume success, given enough time. The DM will determine how much time is "enough" time, based on the circumstances. Pretty simple, but I do have a longer explainer.
And related to the one above, if you attempt an active check, and fail, you cannot retry until you have taken a Long Rest to reconsider your approach. This gets around people who check, fail, and want to check again. And if they fail, want to check again, and so on. Just no.
In some specific cases, rules regarding a skill check need clarification or adjudication. There's only one right now, but there will likely be others, feel free to send them to me.
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