the Joseon period. Although its poetic form was established in the late Goryeo period, it did not become popular until the Joseon period. Many of the sijo reflected Confucian thought; the theme of loyalty is common. Sijo are characterized by a structure of three stanzas of four feet each. Each foot contains three to four syllables except on the third stanza, where the 1st foot is supposed to have 3 syllables and the 2nd foot can have as many as seven. Sijo are thought to have been popular with common people. Gasa is a form of verse, although its content can include more than the expression of individual sentiment, such as moral admonitions. Gasa is a simple form of verse, with twinned feet of three or four syllables each. Some regard gasa a form of essay. Common themes in gasa were nature, the virtues of gentlemen, or love between man and woman. Prose Korean prose literature can be divided into narratives, fiction, and literary miscellany. Narratives include myths, legends, and folktales found in the written records. The principal sources of these narratives are the two great historical records compiled in Classical Chinese during the Koryo era: Samguk sagi (1146; "Historical Record of the Three Kingdoms") and Samguk yusa (1285; "Memorabilia of the Three Kingdoms"). The most important myths are those concerning the Sun and the Moon, the founding of Korea by Tangun, and the lives of the ancient kings. The legends touch on place and personal names and natural phenomena. The folktales include stories about animals; ogres, goblins, and other supernatural beings; kindness rewarded and evil punished; and cleverness and stupidity. Because the compiler of the Samguk yusa was a Zen master, his collection includes the lives of Buddhist saints; the origin of monasteries, stupas, and bells; accounts of miracles performed by Buddhas and bodhisattvas; and other tales rich in shamanist and Buddhist elements. It also includes the 14 hyangga mentioned above. The compilations made in the Koryo period preserved the stories of prehistoric times, of the Three Kingdoms, and of the Silla dynasty and have remained the basic sources for such material. Later compilations made during the Yi dynasty served as a major source of materials for later Yi dynasty fiction. Fiction Korean fiction can be classified in various ways. First, there is Korean fiction written in Chinese and that written in Korean. Second, there are the short works