Final answer:
Sir Gawain and the Green Knight is written in alliterative verse, which utilizes the repetition of initial consonant sounds in closely connected words to enhance the poem's narrative form and medieval setting, in contrast to rhyming couplets and other poetic forms.
Step-by-step explanation:
Sir Gawain and the Green Knight is said to be written in alliterative verse because the poem employs the repetition of the same consonant sounds at the beginning of words that are closely connected. Unlike rhyming couplets, such as in Pope's An Essay on Criticism, alliterative verse focuses on alliteration rather than rhyme.
The structure of Sir Gawain and the Green Knight is particularly complex, with patterns that might be described using an alphabetical formula to show the recurrence of sounds, but this says little about the interplay between the sounds and the meaning of the words.
Moreover, the poem's use of alliteration contributes to the sensuality and physical experience of reading it, enhancing its narrative form and setting.
Other poetic forms, for example, the heroic couplets, show a different style of construction, such as end-stopped lines that create balance and proportion; alliterative verse serves a different purpose in engaging the listener, emphasizing the sound and texture of the language, which well suits the poem's medieval setting.