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I have seen many questions on the web related to Jack of All Trades, revealing facts new to me such as that it adds to initiative, on account of being an ability check which Jack of All Trades indeed affects.

However, one question I haven't been able to find an answer to is whether Jack of All Trades affects AC. If we compare it to initiative, it seems like AC should be an ability check as well:

  1. It uses an ability to arrive at the final value (dex)
  2. It's possible to add proficiency bonus to the check (defensive duelist)
  3. It's a check that is checked for each turn -- it's not a static value since different effects can affect it

Does Jack of all trades affect AC? Is there an official ruling on this?

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    \$\begingroup\$ Hey all, votes aren't up for debate. People can vote how they think they should vote, and we're not going to hold a roundtable discussion to figure out how people are “supposed” to vote on a question. If an asker is wondering what's wrong with their question, feel free to suggest concrete improvements (which is more productive than debates on whether a Q is up-vote or down-vote worthy). I know answering “why is this being downvoted?” is innocent, but it's easy to slide into debates unless the response is a solid suggestion for improvement. \$\endgroup\$ – SevenSidedDie Sep 7 '17 at 15:43
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No. Armor Class is not an ability check.

The PHB does not describe armor class(AC) as an ability check. Armor class is its own in-game thing.

Armor protects its wearer from attacks. The armor (and shield) you wear determines your base Armor Class. (PHB, p. 145)

While the Dexterity ability adds or subtracts to AC in some cases via the Dexterity modifier, it is not universal:

  1. If you wear heavy armor, your Dexterity isn't added to your AC.

  2. If you wear medium armor, there is a limit (2, or 3 with the Medium Armor Mastery feat) to how much dexterity modifies your AC.

  3. If you wear light armor or no armor, your full Dexterity modifier is applied to determine your armor class. (Armor Table: PHB, p. 145).

    The game term used is Dexterity modifier, not a Dexterity based ability check. Further that point, your AC is generally a static value (with specific exceptions depending on feats, cover, magic items, using your shield or not, etc. as you noted). You don't roll a d20 to determine your AC, whereas for an ability check you roll a d20 against a DC to determine success or failure.

Ability Checks (PHB p. 174)

An ability check tests a character's or monster's innate talent and training in an effort to overcome a challenge. The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure. When the outcome is uncertain, the dice determine the results. (PHB p. 174)

@T.J.L. made the following point in a comment that may aid in understanding:

Ability checks, Attack rolls, and Saving throws all involve rolling a d20+modifiers. AC, while not a static value, is (1) not a bonus and (2) is never part of a roll: it is a target that a roll tries to meet.

If you are trying to view AC vis a vis ability checks, AC more closely resembles a DC(Difficulty Class) that an ability check tries to beat.

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    \$\begingroup\$ "If you are trying to view AC vis a vis ability checks, AC more closely resembles a DC(Difficulty Class) that an ability check tries to beat." That is a very good way to describe it. Great answer \$\endgroup\$ – Luke Sep 8 '17 at 5:27

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