A narrative poem often set to music, commonly with four-line stanzas rhyming abab or abcb with lines 1 and 3 in tetrameter and lines 2 and 4 in trimeter. The form was popular in the British Isles in the medieval period and later spread to North America and Australia. Some non-narrative poems are also written in traditional ballad stanzas, and some later ballads extended the traditional stanza to six lines.