Persian metres are patterns of long and short syllables in Persian poetry. Over the past 1000 years the Persian language has enjoyed a rich literature, especially of poetry. Until the advent of free verse in the 20th century, this poetry was always quantitative—that is the lines were composed in various patterns of long and short syllables. The different patterns are known as metres (US: meters). A knowledge of metre is essential for someone to correctly recite Persian poetry—and also often, since short vowels are not written in Persian script, to convey the correct meaning in cases of ambiguity. It is also helpful for those who memorise the verse.