Metre in classical Arabic poetry
Rajaz (, literally 'tremor, spasm, convulsion as may occur in the behind of a camel when it wants to rise') is a metre used in classical Arabic poetry. A poem composed in this metre is an "urjūza". The metre accounts for about 3% of surviving ancient and classical Arabic verse. Some historians believe that rajaz evolved from saj'.
Form.
This form has a basic foot pattern of | ⏓ ⏓ ⏑ – | (where '–' represents a long syllable, '⏑' a short syllable, and '⏓' a syllable that can be long or short), as exemplified through the mnemonic ("Tafā'īl") ' (). Rajaz lines also have a catalectic version with the final foot | ⏓"' – – |.
The form of each (metron) may be ⏑ – ⏑ –, – – ⏑ –, or – ⏑ ⏑ –; only rarely ⏑ ⏑ ⏑ –.
Lines are most often of three feet (trimeter), but can also be of two feet (dimeter). Thus the possible forms are:
| ⏓ ⏓ ⏑ – | ⏓ ⏓ ⏑ – | ⏓ ⏓ ⏑ – | (trimeter)
| ⏓ ⏓ ⏑ – | ⏓ ⏓ ⏑ – | ⏓ – – | ( trimeter catalectic) 
| ⏓ ⏓ ⏑ – | ⏓ ⏓ ⏑ – | (dimeter)
| ⏓ ⏓ ⏑ – | ⏓ – – | (dimeter catalectic)
Uniquely among the classical Arabic metres, rajaz lines do not divide into hemistichs. The early Arab poets rhymed every line on one sound throughout a poem. A popular alternative to "rajaz" poetry was the "muzdawij" couplet rhyme, giving the genre called "muzdawija".
Although widely held the oldest of the Arabic metres, rajaz was not highly regarded in the pre- and early Islamic periods, being seen as similar to (and at times indistinguishable from) the rhymed prose form "saj"'. It tended to be used for low-status, everyday genres such as lullabies, or for improvisation, for example improvised incitements to battle.
Rajaz gained in popularity towards the end of the Umayyad period, with poets al-‘Ajjāj (d. c. 91/710), Ru‘ba (d. 145/762) and Abū al-Najm al-‘Ijlī (d. before 125/743) all composing long "qaṣīda"-style pieces in the metre. Abū Nuwās was also particularly fond of the form.
In the twentieth century, in response to the aesthetics of free verse, "rajaz", both in traditional form and more innovative adaptations, gained a new popularity in Arabic poetry, with key exponents in the first half of the century including poets ‘Ali Maḥmūd Ṭāhā, Elias Abu Shabaki, and Badr Shakir al-Sayyab (cf. his 'Unshūdat al-Maṭar'). Since the 1950s free-verse compositions are often based on rajaz feet.
Example.
A famous, early example is the following incitement to battle by Hind bint Utbah (6th/7th century CE), showing the form | ⏓ ⏓ ⏑ – | ⏑ – ⏑ – |, with the first two elements mostly long, and the fifth one always short:
Relationship to Sarī‘.
The rajaz metre is very similar to the sarī‘, of which the first two metra are the same as rajaz, but the third is shortened: 
| ⏓ ⏓ ⏑ – | ⏓ ⏓ ⏑ – | ⏓ ⏑ – | (trimeter)
| ⏓ ⏓ ⏑ – | ⏓ ⏓ ⏑ – | – – | ( trimeter catalectic) 
Unlike the rajaz, sarī‘ is used in couplets.
The third metron is usually – ⏑ –, ⏑ ⏑ – being very rare, especially at the end of a couplet. 
The two metres are considered by some scholars to be variations of the same metre.
References.
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