The rules by which alliterative verse was composed in Middle English are unclear and have been the subject of much debate. No metrical rules were written down at the time, and their details were quickly forgotten once the form died out: Robert Crowley, in his 1550 printing of "Piers Plowman", simply stated that each line had "thre wordes at the least [...] whiche beginne with some one letter", assuring readers that "this thinge noted, the miter shal be very plesaunt to read". Verse of the Alliterative Revival broadly adheres to the same pattern shown in Old English poetry; a four-stress line, with a rhythmic pause (or caesura) in the middle, in which three of the stresses alliterate, i.e. "aa / ax". However, there are very significant differences. Amongst the features differentiating the Middle English alliterative style from its predecessor is that the lines are longer and looser in rhythm, and the medial pause is less strictly observed, or often absent entirely; hundreds of rhythmic variations seem to have been permitted. While Old English poetry generally employed a clear syntactic break in the middle of the line, in Middle English the line is generally a complete syntactic unit: some poets composed sentences extending over several lines. An example of this style is shown by a few lines from "Wynnere and Wastoure":  There has been much debate on the subject of how lines containing more than two alliterating syllables before the medial pause, which are common in verse of the Revival, should be read. While some scholars have described these additional syllables as a "minor chief syllable" or as having "secondary stress", they have also been interpreted as not altering the four-stress pattern while still contributing to the effect of the line. Some of the most recent analysis proposes that the traditional four-stress model of both Old English and Middle English alliterative verse is a "misapprehension" and that a focus on other apparent rules clarifies the evolution of the form, with Layamon's "Brut" emerging as a key text in the development of alliterative verse. A second type of verse combining rhymed stanzas, usually of thirteen or occasionally fourteen lines, with the basic four-stress line also appeared during the Revival: it appears to have been a new development of the 14th century. Here the alliteration may often follow the pattern "aa / aa", "ax / aa", or even "aa / bb", though lines with four alliterating words are much more common than in verse using the unrhymed long line.