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A rogue has light armor proficiency, but I'm looking to get medium armor.

What is the drawback to using armor you aren't proficient with?

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Wearing non-proficient armor or shield incurs a penalty to attack rolls, STR ability checks, and DEX ability checks.

Each armor or shield has an Armor Check Penalty (or ACP), which is at least 0. The total ACP of your worn equipment is usually applied as a penalty to STR-based and DEX-based skill checks, such as Climb or Stealth.

If you aren't proficient with the armor you're wearing, then the ACP is also applied to your attack rolls (including combat maneuvers), STR ability checks, and DEX ability checks (including your initiative rolls). According to the core rules:

An armor check penalty applies to all Dex– and Strength-based skill checks. A character’s encumbrance may also incur an armor check penalty.

Shields: If a character is wearing armor and using a shield, both armor check penalties apply.

Nonproficient with Armor Worn: A character who wears armor and/or uses a shield with which he is not proficient takes the armor’s (and/or shield’s) armor check penalty on attack rolls as well as on all Dex– and Str-based ability and skill checks. The penalty for non-proficiency with armor stacks with the penalty for shields.

Note that there are exceptions, such as abilities that reduce an item's ACP. If the total ACP is 0, then these drawbacks are effectively negated, although arcane casters may still suffer spell failure chance.

(thanks to KRyan and Ifusaso for the clarification)

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  • \$\begingroup\$ From your link, “[Mithral’s] decrease [in armor weight class] does not apply to proficiency in wearing the armor.” It’s blitheringly stupid, but Paizo decided that mithral needed nerfing and so prevented mithral from being used to wear heavier-weight-class items. I recommend ignoring it in your games, but we can’t ignore it in answers. Also, attack rolls are not the only thing affected by nonproficiency ACP—Strength and Dexterity ability checks are also added to the list of things penalized. This is particularly important because Initiative is a Dexterity ability check. \$\endgroup\$ – KRyan Aug 2 '19 at 2:47
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    \$\begingroup\$ It may be worth dropping in a line that this means there is no "effective" penalty (outside ASF) for wearing non-proficient armor with 0 ACP. Many an animal companion has had Darkleaf Hide barding... \$\endgroup\$ – Ifusaso Aug 2 '19 at 11:58

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