Arabic poetry ( ash-shi‘ru al-‘Arabīyyu) is the earliest form of Arabic literature. Present knowledge of poetry in Arabic dates from the 6th century, but oral poetry is believed to predate that. Arabic poetry is categorized into two main types, rhymed or measured, and prose, with the former greatly preceding the latter. The rhymed poetry falls within fifteen different meters collected and explained by al-Farahidi in The Science of ‘Arud. Al-Akhfash, a student of al-Farahidi, later added one more meter to make them sixteen. The meters of the rhythmical poetry are known in Arabic as "seas" (buḥūr). The measuring unit of seas is known as "taf‘īlah," and every sea contains a certain number of taf'ilas which the poet has to observe in every verse (bayt) of the poem. The measuring procedure of a poem is very rigorous. Sometimes adding or removing a consonant or a vowel can shift the bayt from one meter to another. Also, in rhymed poetry, every bayt has to end with the same rhyme (qāfiyah) throughout the poem. Al-Kʰalīl b. ˀAḫmad al-Farāhīdī (711 – 786 A. D.) was the first Arab scholar to subject the prosody of Arabic poetry to a detailed phonological study. He failed to produce a coherent, integrated theory which satisfies the requirements of generality, adequacy, and simplicity; instead, he merely listed and categorized the primary data, thus producing a meticulously detailed but incredibly complex formulation which very few indeed are able to master and utilize. Researchers and critics of Arabic poetry usually classify it in two categories: classical and modern poetry. Classical poetry was written before the Arabic renaissance (An-Nahḍah). Thus, all poetry that was written in the classical style is called "classical" or "traditional poetry" since it follows the traditional style and structure. It is also known as "vertical poetry" in reference to its vertical parallel structure of its two parts. Modern poetry, on the other hand, deviated from classical poetry in its content, style, structure, rhyme and topics. Pre-Islamic poetry One of the first major poets in the pre-Islamic era is Imru' al-Qais, the last king of the kingdom of Kinda. Although most of the poetry of that era was not preserved, what remains is well regarded as among the finest Arabic poetry to date. In addition to the eloquence and artistic value, pre-Islamic poetry constitutes a major source for classical Arabic language both in grammar and vocabulary, and as a