Classical Persian poetry is always rhymed. The principal verse forms are the Qasideh, Masnavi, Qazal and Ruba'i. The qasida or ode is a long poem in monorhyme, usually of a panegyric, didactic or religious nature; the masnavi, written in rhyming couplets, is employed for heroic, romantic, or narrative verse; the ghazal (ode or lyric) is a comparatively short poem, usually amorous or mystical and varying from four to sixteen couplets, all on one rhyme. A convention of the ghazal is the introduction, in the last couplet, of the poet's pen name (takhallus). The ruba'i is a quatrain with a particular metre, and a collection of quatrains is called "Ruba'iyyat" (the plural of ruba'i). Finally, a collection of a poet's ghazals and other verse, arranged alphabetically according to the rhymes, is known as a divan.
A word may not be out of place here on the peculiar difficulties of interpreting Persian poetry to the western reader. To the pitfalls common to all translations from verse must be added, in the case of Persian poetry, such special difficulties as the very free use of Sufi imagery, the frequent literary, Koranic and other references and allusions, and the general employment of monorhyme, a form highly effective in Persian but unsuited to most other languages. But most important of all is the fact that the poetry of Persia depends to a greater degree than that of most other nations on beauty of language for its effects. This is why much of the great volume of "qasidas in praise of princes" can still be read with pleasure in the original, though It is largely unsuited to translation. In short, the greatest charm of Persian poetry lies, as Sir E. Denison Ross remarked, in its language and its music, and consequently the reader of a translation "has perforce to forego the essence of the matter".