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.