In his annotated bibliography English Versification, 1570–1980, Terry V.F. Brogan delivered a damning verdict on the discipline of Middle English:
In comparison with the study of OE verse, which is a discipline relatively coherent in its terms and its methods, our understanding of ME verse is far less advanced. One wonders why those departments charged with the study of the English language and its poetry have allowed this situation to persist […] Much work needs to be done. The study of ME verse may be as much as a century behind that of Old English.
Since this was written, certain areas of Middle English (I think especially of alliterative metre) are no longer so far behind, but it is surely still true that ‘[m]uch more work needs to be done’, perhaps especially by those of us working on Middle English romances, which present us with an impressive variety of verse forms. Leaving aside the more familiar forms—the rhyming couplets and tail-rhyme, each itself encompassing diverse species —we find the following:
• aaabab stanza (Octavian, Southern Version)
• abab stanza (e.g. Sowdon of Babylon, Apollonius of Tyre fragment)
• septenaries with medial and end rhyme (Ashmole Sir Ferumbras)
• abababab stanza (Stanzaic Morte Arthur and prologue to Thomas of Erceldoune)
• abababab+cbc stanza (Sir Tristrem) [+ indicates that the stanza ends in a wheel]
• abababab+cdddc stanza (e.g. Awntyrs off Arthure)
• alliterative long lines (e.g. Wars of Alexander, Alliterative Morte Arthure)
• stanza consisting of alliterative long lines+ababa (Sir Gawain and the Green Knight)
• the rhyme-royal stanza (e.g. Generides, Trinity College Cambridge MS)
In reality, the variety is greater still, for this is a simplified classification which largely ignores not only rhythmical distinctions but also variations of stanza forms within some of the romances mentioned. For example, the Ashmole Sir Ferumbras shifts from septenaries to tail-rhyme at line 3411, 4 and Sir Tristrem, as it stands in the Auchinleck manuscript, also has stanzas rhyming abababab+cac (stanzas beginning at lines 34, 67, 78, 244, 1178, 2168), along with various anomalous stanza forms which are probably scribal in origin.