There is a very interesting discussion going on over at Dreams in the Lich House, which I'm unable to join in on because I don't do the Google+ thing. It's a shame, it's not the first discussion over there I've wanted to get involved in, it is a very fine blog, but having to set up yet another profile for something is at the moment a step too far for me (I have dozens already and resent the fact I have so many).
It relates to something I've encountered before, most acutely when trying to create a viable Early- (as opposed to High-)Medieval background for my 'Dark Age' campaign, and therefore I think it will impact on Jens D's 'Lost Songs of the Nibelungs' project. It was a very early set of things I was playing with back in the day, maybe around 1983, because it was covered in an article in an early White Dwarf, called 'Dungeons and ... Dragoons?', which provided some troop-types for non-European and non-Medieval troops - Ancient Greeks and Assyrians, Han Chinese, Aztecs etc. Also, as the proud possessor of Deities and Demigods, I wanted to use Achilles in my games as much as King Arthur and The Grey Mouser.
To what extent is 'plate mail' a synonym for 'the best armour you can get'? That is the question, really. If, like 'Dungeons & ... Dragoons?' you assume that medieval plate co-exists with Hoplite armour and lorica segmentata, then it seems pretty obvious that these forms of armour are not as good as full plate. But what if, as in an Early Medieval setting, some old Roman armour is probably the best you can get? Is this not then the functional equivalent of plate? John at Dreams in the Lich House poses this as the difference between an 'absolute' and a 'relative' scale of armour - absolute plate is 'plate mail is the best, lorica segmentata is not as good as plate, lorica segmentata is not as good as the best', relative plate is 'plate mail is the best, lorica segmentata is the best, lorica segmentata is as good as plate mail'.
My fix was to assume that in the equipment list and for the sake of calculating AC, 'plate mail' meant 'the best Roman armour'. Sure, there were a few weapons that didn't quite fit, but so what? It's a fantasy game. Why shouldn't my players decide they wanted rather a-historic massively-long swords if they wanted (most didn't, not liking giving up shields to wield them)?
For a game that has a fixed and fairly coherent background this makes sense I think - for my Arthurian campaign, for Jens's Siegfried-era campaign, for John's Heroic/Classical Greek campaign. There are no outside influences, no Richard IIIs popping up in C15th plate armour to fight Achilles. So there's no reason not to assume that 'plate mail' stands for 'the best you can get'. But if better armour becomes available (maybe someone in the C5th makes some field-plate? Maybe a portal to another world brings metal-sheathed behemoths into the field?) then the definitions need to change. Lorica segmentata then becomes 'better than chain, not as good as plate' and I'd assign it to an AC of 4 (because I'm old-school), and probably the breastplates of Greek warriors I'd call 'equivalent to chain' (ie, AC5).
But it is a tricky business taking the implied background away and trying to do something else.