A Welsh form, the Cywydd Deuair Fyrion is a couplet based stanza form. Each line consists of 4-syllables and the couplet carry an aa rhyme, Multiple couplets can be combined into stanzas or a poem.
Example rhyme schemes
aa bb cc dd
aabbccdd
aabb cc ddee ff
and so on
Example
Tylwyth Teg by Jez Farmer
The fairest fae
Or so they say
Longs for a child
Blonde hair and mild
If she sees one
He’ll soon be gone
Replaced by one
Changeling son
The fairest fae
Or so they say
She’ll wed a man
Fairy wife can
Never undo
The fae taboo
Iron brings pain
Ne’er seen again
The fairest fae
Or so they say
Bound by fae lore
Touched iron ore
Is sent back home
Never hear from
So, tells the man
In tales of Bran
A Garret Poet 