Byr a Thoddaid

Byr a Thoddaid is a four line stanza made up of a couplet containing 8 syllables per line a line has 10 syllables followed by a line of 6 syllables. The couplets may come in either order: 8, 8, 10, 6 or 10, 6, 8, 8. 
This is unaccented verse. The syllables are not stressed. 

The 8-syllable couplet rhymes. In the other couplet, the main rhyme occurs before the end of the 10-syllable line and at the end of the 6-syllable line. In addition, the syllables that follow the main rhyme in the first line of the couplet are linked with the beginning of the 6-syllable line by alliteration, assonance, or secondary rhyme. The poem may end with a couplet. 

In the schematic that follows, ordinary syllables are marked by the letter x, main rhymes by capital letters, and alliteration, assonance, or secondary rhyme by small letters. 

Pattern 1: 

1. xxxxxxxA 
2. xxxxxxxA 
3. xxxxxxxBxc 
4. xcxxxB 

Pattern 2: 

1. xxxxxxxAxb 
2. bxxxxA 
3. xxxxxxxC 
4. xxxxxxxC 

Byr a Thoddaid

Byr a Thoddaid is a Welsh syllabic quatrain form consisting of 2 couplets, either of which can appear as the first couplet of the stanza. 

One couplet is two 8-syllable lines that rhyme with each other. The other couplet consists of one 10-syllable line and one 6-syllable line.

The 10-syllable line has a rhyme before the end in the 7^th, 8^th or 9^th syllable that rhymes with the end of the 6-syllable line. 

Syllables after the rhyme correspond in some way to the beginning of the 6-syllable line. This corresponence might be alliteration, rhyme or assonance.

Scheme:

A byr a thoddaid verse might look like:

xxxxxxxa
xxxxxxxa
xxxxxxxbx1
x1xxxb

or

xxxxxxxbx1
x1xxxb
xxxxxxxa
xxxxxxxa

where the 1 represents the association through 
assonance, alliteration, or rhyme. 
The “a” and “b” are the more normal rhymes.