Korean literature is the body of literature produced by Koreans, mostly in the Korean language and sometimes in Classical Chinese. For much of Korea's 1,500 years of literary history, it was written in Hanja. It is commonly divided into classical and modern periods, although this distinction is sometimes unclear. Korea is home to the world's first metal and copper type, the world's earliest known printed document and the world's first featural script. Korean literature Classical Korean literature has its roots in traditional folk beliefs and folk tales of the Korean peninsula. There are four major traditional poetic forms: hyangga ("native songs"); byeolgok ("special songs"), or changga ("long poems"); sijo ("current melodies"); and gasa ("verses"). Other poetic forms that flourished briefly include the kyonggi-style, in the 14th and 15th centuries, and the akchang ("words for songs") in the 15th century. The most representative akchang is Yongbi och'on ka (1445–47; Songs of Flying Dragons), a cycle compiled in praise of the founding of the Yi dynasty. Korean poetry originally was meant to be sung, and its forms and styles reflect its melodic origins. The basis of its prosody is a line of alternating groups of three or four syllables, which is probably the most natural rhythm to the language. One famous earliest poetry or lyric song was the Gonghuin (Konghu-in) by Yeo-ok during Gojoseon. Hyangga Hyangga (hangul: 향가, hanja: 鄕 歌) was written in Korean using modified hanja in a system that is called idu (hangul: 이두, hanja: 吏 讀), literally "clerk's writings." Specifically, the variety of idu used to write hyangga was sometimes called "hyangchal". Idu was a system using hanja characters to express Korean. The key to the system was to use some hanja characters for their intended purpose, their meaning, and others for their pronunciation, ignoring their pictographic meaning. On the surface, it appears to be a complicated, even incomprehensible system, but after using the system one becomes comfortable with certain characters consistently standing for Korean words. Hyangga was the first uniquely Korean form of poetry. It originally indicated songs that were sung in the Silla period. Only twenty five survive. The Samguk Yusa contains 14 poems and the "Gyunyeojeon", a set of biographies of prominent monks, contains eleven poems. Both these classic works were written much after the Unified Silla, in the subsequent Goryeo dynasty, yet the poems in the Samguk Yusa appear to be based on no-longer-extant