The Shape of the B-Verse in Middle English Alliterative Poetry.
PPl is the largest exception to what appears to be the authorial (rather than merely scribal) practice in ME alliterative poetry in which the b-verse consists of two lifts and one to three dips, with a minimum of four and a maximum of nine syllables. PPl also a major exception to the distributional rule dictating that if two or three dips are filled only one can have two or more syllables.
Speculum 61 (1986): 564-92, esp. 577-83.
Duggan, Hoyt N.