Arūḍ (Arabic: اَلْعَرُوض al-ʿarūḍ) is the study of poetic meters, which identifies the meter of a poem and determines whether the meter is sound or broken in lines of the poem. It is often called the Science of Poetry (Arabic: عِلْم اَلشِّعْر ʿilm aš-šiʿr). Its laws were laid down by Al-Khalīl ibn Aḥmad al-Farāhīdī (d. 786), an early Arab lexicographer and philologist. In his book Al-ʿArḍ (Arabic: العرض), which is no longer extant, he described 15 types of meter. Later Al-Akhfash al-Akbar described a 16th meter, the mutadārik.
Following al-Khalil, the Arab prosodists scan poetry not in terms of syllables but in terms of vowelled and unvowelled letters, which were combined into larger units known as watid or watad (“peg”) and sabab (“cord”). These larger units make up feet (rukn, pl. arkān).