What ability would Poetry fall under?
(relevant) Language?
Craft Poems?
???
Not Language - it is an art beyond the mere ability to understand idioms and have a broad vocabulary. But the best poets would have Language 6 (or at least 5 (spec: Poetry)). (Anything less might be pleasing to the peasant's ear, but dismissed as "doggerel" by the educated. "There once was a mage from Val Negra...")
It might be tempting to toss it in with Arts Liberales - let's say it's "not unrelated" to the study of Rhetoric, but probably not a direct function of AL. (In oral traditions, even "peasant" traditions in literate societies, a knowledge of letters or etc. is not a prerequisite.)
Saying that it's a separate "Craft" seems to demean the Art of Poetry, but that might be the best fit given the game mechanics. It is indeed its own pursuit, and the uneducated can lay claim to it equally, so whether craft, art or profession, it's the same rose by any other name.
Despite my appreciation of the fact that the true nature of poetry goes far beyond this, mechanically, poetry is covered by a Language specialization. You may need other skills (Artes Liberales or Charm, or something else at the storyguide's discretion) to actually deliver the poem well across your preferred medium, however.
A&A sets writing poetry as a Craft ability. It also requires a score of 5 in the relevant language. 4+ specialty is probably ok to.
Aha! I knew I'd read something, but couldn't remember what (or where) that 'something' was!
Thanks!
Actually, by the Core Rules, MB's right - p. 66 col i, last par: "(Living Language) also covers artistic composition in the language..."
I have to say I like it as a separate Craft much better. I know people who write poetry - it's more than a vocabulary, and not (always) dependent on one either.
You can always do Profession: Poet or Profession: Bard and say that you need at least Home language 5 or Other language 6 to avoid getting penalties
I'm afraid we've had a misunderstanding. Was my grammar incorrectly applied? I was attaching "mechanically" to "poetry is covered by a language specialization." I was not trying to say that it goes further mechanically. In actuality, we are in agreement on every point. 
Again, sorry if my sentence wasn't grammatically correct.
Now I'm confused - you were referring to "game mechanics", not "linguistic mechanics", right? I was simply commenting that a) that is correct by the core rules, and b) I like the A&A ruling better (which I was not aware of either).
This is also confirmed by the statistics of the "wandering emir" Mansur, a Companion from Semita Errabunda, whose "Enchanting Poetry" Virtue is described to work "As Enchanting Music Virtue, only applies to Mansur’s Ability to sway others with his poetry (using Living Language skills)." I seem to remember there's something along these lines in the Bedouin section of tC&tC, too.
I think it's not a correct argument the one that goes "(Living Language) only covers vocabulary, therefore it cannot cover artistic expression, which is a separate thing." Precisely because we are told that (Living Language) covers not only vocabulary, but also training in artistic expression. It's important to keep in mind in this sense that "art" in the middle ages was much less freeform, and generally considered much more about training and knowledge than about "genius".
The Contested Isle has a section on Bards on pages 115-7. They use Profession:Storyteller or Profession:Poet, and those with the Touched by (Realm) virtue can create magic stories or poems much like a crafter can create magic items.
We are "told" that in the context of the rules of a game - my opinion that you quote is from my knowledge of language in the real world. Others are welcome to other opinions, but pardon me if I feel some game rule does nothing to refute the "correctness" of my beliefs.
Artistic expression and comprehension/vocabulary/communicative competence are connected only in that they both use language, nothing more imo (and in the opinion of both formal students of linguistics and artists generally recognized as "poets").
"Genius" *(as an absolute measure, rather than the relative "genius" that lies within all humans) would be covered by the "Free Expression" virtue.
And since "(Living Language" has nothing to do with formal knowledge or training (which is why the least educated can pick it up by informal exposure), and Craft(X) would, that statement supports the A&A ruling.
In short, any fool can find 2 words that rhyme within their vocabulary. People with larger vocabularies can find more words that rhyme, and more impressive words. Poetry is far, far more than finding impressive words that rhyme.