The Byr a Thoddaid is a Welsh quatrain form. Each quatrain is divided into two combined couplets. One couplet is constructed of 8-syllable lines with an end rhyme. The other couplet is constructed of a 10-syllable line followed by a 6-syllable line. The 6-syllable line ends with a sound echo of a word in the 10-syllable line and contains a sound echo of the end word of the 10-syllable line
Schematic
xxxxxxxa
xxxxxxxa
xxx(b)xxxxxc
x(c)xxxb
or
xxx(a)xxxxxb
x(b)xxxa
xxxxxxxc
xxxxxxxc
Example
The Challenge of April by Jez Farmer
The challenge a poem per day
Each year NaPoMo comes to stay
The pen must write in rhyme and form, maybe
A free verse if there’s time
A few months yet, to stop and think
What words look right written in ink
All I know I’ll be ready to write
Day or night as words flow
A Garret Poet 