This is one of the simpler Welsh forms. Or, rather, these are some of the simpler Welsh forms, for there is certainly more than one. They are, however, related, and they have some similarities.
One of their features is that can be used in the English Language. Robert Frost (yes, him again) was right when he pronounced that 'poetry is what gets lost in translation', but he was less right in the case of the englyn.
Englynion were originally three-line poems, but soon became four-line. An englyn (plural englynion) simply means a verse of three or four lines written in strict metre.
The 'soldier's englyn'.This comprises three lines of seven syllables each using an end-rhyme. It is straightforward to use, and adapts easily to the English language. This is an example in Welsh:
Pen y borthaf a'm porthes.
Neud anwen nad er fy lles.
Gwae fy llaw, llym ddigones.
This uses a more complex scheme. There are three lines of ten, six, and seven syllables. The rhyme in the first line comes one, two, or three syllables before the end, and is then used at the end in the 'normal' way in lines two and three.
This is my favourite form of Englyn. There are four lines of seven syllables each. The lines do not rhyme in the normal way, but 'proest'. The final vowel sound must differ, while the final consonant remains the same. This is an example in English of my own:
Here beneath the singing wind
bare hillside, the river's end
clouds touch fingers to the ground:
this was where they held their land.
It's an extract from the longer poem, The Cwmwysg Poacher. That one used a variety of rhyme schemes, but does not make any claim to be a good poem. Actually, this is not a good example of the Englyn Proest either, because of the use of 'ground'. Strictly speaking, dipthongs should proest with dipthongs and short vowels with short. But rules are made to be broken, aren't they?
Englyn Unodl Union
When people say 'an englyn', this is the form that they usually mean. There are four lines of ten,six, seven, and seven syllables, all using the same rhyme - though not as end rhyme in the first line. The form uses a todydd byr and a couplet of cywydd deuair hirion.
In the first line, the rhyme is used one, two, or three syllables from the end. The end of the first line must either rhyme or alliterate with the middle of the second. The end-rhymes in the third and fourth lines must not both be of accented and unaccented syllables, and similarly the main rhyme in the first and second syllables must not both be on accented characters.
Complicated? It's not quite as bad as it sounds!
Other Welsh Forms
Cynghanedd is the name for the strict metrical pattern that operates within every single line. Briefly, the cywydd dewair hirion is a poem written in seven-syllabled couplets. This is a simplified explanation - the stressing and unstressing of lines is also important, as is tonal emphasis. For a detailed survey of Welsh poetry, I can't do better than to recommend that you have a look at Welsh Verse, a superb book by Tony Conran, published in several editions by Seren Books.