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Mithral counts as one armor weight category less (heavy, medium, light) for "purposes of movement and other limitations... which are left unspecified.

An example of where this might come up: Armor Crystals; which can only be put on certain weight categories of armors.

What other circumstances will cause this problem, and which way does this interaction fall?

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    \$\begingroup\$ Do you want separate entries for how this works in 3e and 3.5e or is the 3e tag included for completeness? (Typically, the 3.5e tag subsumes the 3e tag, but the 3e tag gets used when a question's specifically about that edition or, sometimes, as a reminder when a 3.5e question relies heavily on a text from that edition.) \$\endgroup\$ – Hey I Can Chan Jul 20 '16 at 19:24
  • \$\begingroup\$ @nijineko I’d forgotten about this entirely until it was recently brought up. I’ve updated my answer pretty substantially, but I can’t recall if you get a notification on an edit to an answer, so I’m doing that manually. Is there anything else you’d like to see in an answer? \$\endgroup\$ – KRyan Feb 14 '18 at 2:34
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All purposes, period. Say an armor crystal can only go on light armor: does that fall under the category of “movement or other limitations”? Yes, it does: it’s a limitation on the armor’s ability to carry this armor crystal. Anything that is restricted to a particular weight class of armor is affected by mithral.

In effect, mithral breastplate is a light armor, not a medium one. Mithral full-plate is a medium armor, not a heavy one. The only time an armor doesn’t count as a weight class lower is when the armor was light to begin with (and thus it never counts as a weight class lower).

This consensus on this ruling is incredibly wide; I have literally never heard anyone suggest it be otherwise. The description of mithral is somewhat vague (it counts as lighter “for other purposes”) but ultimately we have a vague but exceedingly broad characterization, and then we have absolutely no suggestion anywhere of any kind of exception.

For what it’s worth, and noting the problems with the FAQ, the FAQ agrees:

Is a character proficient with light armor, such as a rogue, considered to be proficient with mithral breastplate? What about a character proficient with medium armor, such as a barbarian—is he considered proficient with mithral full plate armor?

The description of mithral on page 284 of the Dungeon Master’s Guide is less precise than it could be in defining how it interacts with armor proficiency rules. The simplest answer— and the one that the Sage expects most players and DMs use— is that mithral armor is treated as one category lighter for all purposes, including proficiency. This isn’t exactly what the Dungeon Master’s Guide says, but it’s a reasonable interpretation of the intent of the rule (and it’s supported by a number of precedents, including the descriptions of various specific mithral armors described on page 220 of the Dungeon Master’s Guide and a variety of NPC stat blocks).

Thus, a ranger or rogue could wear a mithral breastplate without suffering a nonproficiency penalty (since it’s treated as light armor), and each could use any ability dependent on wearing light or no armor (such as evasion or the ranger’s combat style). A barbarian could wear mithral full plate armor without suffering a nonproficiency penalty (since it’s treated as medium armor), and he could use any ability dependent on wearing medium or lighter armor (such as fast movement).

The same would be true of any other special material that uses the same or similar language as mithral (such as darkleaf, on page 120 of the EBERRON Campaign Setting).

So on the one hand, we have

  1. actual text, though it is somewhat vague,

  2. every example to be found anywhere in the system, including NPC statblocks, descriptions of specific armors, and the actual presence of “mithral breastplate” listed under light armor in Races of the Wild,

  3. the opinion of whoever was writing the FAQ, and

  4. the extremely wide consensus of D&D 3.5e players.

On the other hand, we have... well, we have that the wording is kind of vague, so maybe it doesn’t actually cover some cases. Which, fine—something could easily say “this only works in light armor, and mithral medium armor doesn’t count.” The rules would allow such an exception. But no such exception is written anywhere in the system (and it would be dumb if it did).

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  • \$\begingroup\$ But would an armor crystal be considered a "limitation" with regards to the armor? The "movement and other limitations" only comes into play when the character is potentially limited by the armor, I'm not entirely convinced that it somehow applies to armor augments? And all light armor crystals can be applied to any weight category of armor. It would be the ones that can only be applied to medium or heavy that would call out this interaction. I think "other limitations" means more skill checks like swimming. And medium mithral armor is medium armor that counts as light, it isn't actual light. \$\endgroup\$ – nijineko Jul 20 '16 at 19:32
  • \$\begingroup\$ @user30200 It’s a limitation on the armor: its weight class limits its ability to use an armor crystal. A mithral breastplate could not use an armor crystal that works only on medium and heavy armors. There is no indication in the rules that the armor fails to count as anything but light for any circumstance. That it is “counts as light but isn’t actually light” is meaningless when it always counts as light for everything, and never counts as medium for anything, which is the situation we have here. \$\endgroup\$ – KRyan Jul 20 '16 at 19:35
  • \$\begingroup\$ @user30200 The MIC has no armor crystals that're light armor exclusive and only the iron ward diamond (MIC 26) is limited to medium and heavy armors. Are there other armor crystals elsewhere? \$\endgroup\$ – Hey I Can Chan Jul 20 '16 at 19:38
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    \$\begingroup\$ A bard can use mithral breastplate proficiently, yes. \$\endgroup\$ – KRyan Jul 21 '16 at 11:44
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    \$\begingroup\$ @GMJoe Hm, that’s true. It doesn’t really seem part of the question but it is in the title. \$\endgroup\$ – KRyan Jul 22 '16 at 2:45

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