Final answer:
Sir Gawain and the Green Knight is written in Middle English alliterative meter, a poetic structure that emphasizes alliteration instead of rhyme and is characteristic of Middle English literature from between 1150 to 1500.
Step-by-step explanation:
Sir Gawain and the Green Knight is written in Middle English alliterative meter. This form of poetry, prevalent during the Middle English period between 1150 to 1500, does not rely on rhyme to the same extent as later periods. Instead, it emphasizes the alliterative pattern within each line of verse, akin to the poetic structures found in Old English literature such as "Beowulf."
Examples of Middle English literature, like Geoffrey Chaucer's "The Canterbury Tales", showcase the linguistic and rhythmic patterns of that era. The metrical form used in Sir Gawain and the Green Knight contributed to the stateliness and massive quality of heroic narratives, as mentioned in the provided reference discussing the heroic measure and poetic meter.
Unlike the ballad form with its distinctive ABAB or ABCB rhyme schemes and alternating syllabic length, the alliterative meter focuses on the repetition of the initial consonant sounds within a line. The lack of fixed rhyme scheme distinguishes it from iambic or trochaic tetrameter, typically found in ballads and other narrative poems that postdate the Middle English period.