Final answer:
The poetic device used in Sir Gawain and the Green Knight is a ballad, which is a narrative poem typically written in quatrains with a specific rhyme scheme and syllable count.
Step-by-step explanation:
The poetic device used in the story Sir Gawain and the Green Knight that is a 12 or three syllable word followed by four rhymed lines is called a ballad. Ballads were used to transmit stories orally from one generation to another and often recited with musical accompaniment. They are most commonly written in quatrains with a rhyme scheme of A B C B and contain eight syllables in the first and third lines, and six syllables in the second and fourth lines.