Poetry has been featured extensively in Thai literature, and constituted the near-exclusive majority of literary works up to the early Rattanakosin period (early 19th century). Most of imaginative literary works in Thai, before the 19th century, were composed in poetry. Consequently, although many literary works were lost with the sack of Ayutthaya in 1767, Thailand still has a great number of epic poems or long poetic tales -- some with original stories and some with stories drawn from foreign sources. The Siamese poetical medium consists of five main forms, known as khlong, chan, kap, klon and rai; some of these developed indigenously while others were borrowed from other languages. Thai poetry dates to the Sukhothai period (13th–14th centuries) and flourished under Ayutthaya (14th–18th centuries), during which it developed into its current forms. Though many works were lost to the Burmese conquest of Ayutthaya in 1767, sponsorship by subsequent kings helped revive the art, with new works created by many great poets, including Sunthorn Phu (1786–1855). Prose writing as a literary form was introduced as a Western import during the reign of King Mongkut (1851–68) and gradually gained popularity, though poetry saw a revival during the reign of King Vajiravudh (1910–25), who authored and sponsored both traditional poetry and the newer literary forms. Poetry's popularity as a mainstream form of literature gradually declined afterwards, although it is still written and read, and is regularly employed ceremonially. Forms Thai poetic works follow established prosodic forms, known as chanthalak (, ). Almost all have rules governing the exact metre and rhyme structure, i.e. the number of syllables in each line and which syllable rhymes with which. Certain forms also specify the tone or tone marks of syllables; others have requirements of syllable "heaviness". Alliteration and within-line rhyming are also often employed, but are not required by the rules. Khlong The khlong (, ) is the among oldest Thai poetic forms. This is reflected in its requirements on the tone markings of certain syllables, which must be marked with mai ek (, , ) or mai tho (, , ). This was likely derived from when the Thai language had three tones (as opposed to today's five, a split which occurred during the Ayutthaya period), two of which corresponded directly to the aforementioned marks. It is usually regarded as an advanced and sophisticated poetic form. In khlong, a stanza (bot, , ) has a