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.