From Kenneth Sisam's introduction to Fourteenth Century Verse and Prose Oxford University Press 1921, corrected 1937 (with a Middle English Vocabulary by JRR Tolkien).
About the middle of the [fourteenth] century, imaginative poetry found a new home in the West-Midlands...
They preferred the unrimed alliterative verse, which from pre-Conquest days must have lived on in the remote Western counties without a written record; and for a generation rime is overshadowed...
At the time alliterative verse was fitted to become the medium of popular literature. Prose would not serve, because its literary life depends on books and readers...
It was not easy to write verse that depended on number of syllables, quantity or rime. The fall of inflexions brought confusion on syllabic metres; there were great changes in the quantity and quality of vowels; and these disturbances affected dialects unevenly.
It must have been hard enough for a poet to make rules for himself: but popularity involved the recital of his work by all kinds of men in all kinds of English, when the rimes would be broken and the rhythm lost...
The more fortunate makers of alliterative poems, whose work depended on the stable yet elastic frame of stress and initial consonants, possessed a master key to the dialects.
Examples of Middle English alliterative verse includes two out of three of the great poets of the 14th century (the other being Chaucer): William Langland who wrote Piers Plowman, and the anonymous author of Sir Gawain and the Green Knight and Pearl.
The alliterative metre used descended from Old English, where each line is divided into two half-lines, and each half line has two main stressed syllables - in the first half-line both stresses should alliterate, and these two usually alliterate with the first stress of the second half line.
But sometimes extra alliteration is scattered about, for effect. Perhaps the most alliterative poem ever, seems to have been a piece of apparently 'comic verse' now called 'Blacksmiths'
Swarte-smeked smethes, smattered with smoke,
Drive me to deth with den of here dintes:
Swich nois on nightes ne herd men never,
What knavene cry and clattering of knockes!
The cammede kongons cryen after 'Col! Col!'
And blowen here bellewes that all here brain brestes.
'Huf, puf,' saith that on, 'Haf, paf,' that other.
They spitten and sprawlen and spellen many spelles,
They gnawen and gnacchen, they groan togedire,
And holden hem hote with here hard hamers.
Of a bole hide ben here barm-felles,
Here shankes ben shackeled for the fere-flunderes.
Hevy hameres they han that hard ben handled,
Stark strokes they striken on a steled stock.
'Lus, bus, las, das,' rowten by rowe.
Swiche dolful a dreme the Devil it todrive!
The maistre longeth a litil and lasheth a lesse,
Twineth hem twein and toucheth a treble.
'Tik, tak, hic, hac, tiket, taket, tik, tak,
Lus, bus, las, das.' Swich lif they leden,
Alle clothemeres, Christ hem give sorwe!
May no man for brenwateres on night han his rest.