What was William Langland's contribution to English poetry?

William Langland's The Vision of
William, Concerning Piers the Plowman is in the alliterative style that was
revived during of his period in the mid- to late-1300s. The usual group of readers
during this period were provincial nobles (i.e., nobles living away from the cities of
commerce; living in the provinces). However Langland's work, first published in 1362,
appealed to a broad national group of readers from the rising middle (bourgeoisie)
class, so Piers the Plowman was widely read by high and low clergy
and by lay persons. Therefore, one contribution Langland made was to support and foster
a new national readership that had a taste for didactic, or serious and morally
instructive, literature.

Langland's primary contribution was in
helping to advance the Alliterative Revival of the mid- to late-1300s. Alliterative
verse was the original style of Old English poetry but was a feature of oral traditions
poetry. Langland was one of a few poets, including the Pearl Poet, in the last half (or
third) of the 14th century who reintroduced alliterative poetry, possibly as a reaction
to the Anglo-Norman court that brought rhyming French versification with them. There
were two distinctive features to this reintroduction.

The first is
that in the Alliterative Revival poets like Langland wrote
their alliterative poetry, thus removing it from the confines of oral tradition and
offering it as an alternative to Norman rhyming verse. The second is that Middle English
alliterative verse altered some of the strict rules of style in Old English alliterative
verse. For one thing, the Old English oral style of hemistich with caesura (two half
lines with a medial pause) and four stressed syllables, with hemistich alliteration
governed by the third syllable consonant, was altered. Written alliterative verses might
have three or five or six stressed syllables instead of four, with two instead of one
alliterations in the second hemistich, thus varying the previous Old English norm of two
alliterations, caesura, one alliteration. Langland adheres to this norm in line 1.006:
"How bisie they ben
aboute the maze?" Two alliterations on /b/ come in the
first hemistich before the caesura (i.e., ben // about) and
one comes after in the second hemistich.

Here is an example
of a kind of variation on the form: "And the feld
ful of folk, I shal yow
faire shewe" (1.002). It has four alliterations on /f/,
with three before the caesura and one after. Langland was instrumental in bringing these
changes about and popularizing written alliterative verse.