Regulated verse, a form which was perfected in the eighth century and continued to be the most commonly used verse form until modern times, exploits the characteristic tonality of the Chinese language, using a very rigid formal structure in which tension is created by combining tonal contrast with verbal parallelism and rhyming system.
One feature deemed essential to the perfection of form in the writing of the "regulated-verse poems" is the use of parallelism and antithesis. A "regulated" poem is a poem of eight lines, which can be either heptasyllabic (seven syllables) or pentasyllabic (five syllables), but it is de rigueur for the two middle couplets (lines 3-6) to show parallelism and antithesis.
(Encyclopedia of literary translation into English, Volume 1 By Olive Classe)
The four tones of medieval Chinese are divided into two classes, level and oblique, and only level-tone words may be used as rhymes. Tonal contrast has to be observed throughout the whole poem, in all four couplets. A word in the "even" or ping tone matched by a word in "oblique" or ze tone.
All Regulated Verse poems have eight lines (four couplets) and have the same rhyme throughout in alternate lines, rhyme in the first line being optional.
In the two central couplets there has to be verbal parallelism, that is to say, if you have 'brown cown' in the first line of the couplet, you must have some matching expression such as 'white horse' in the same position in the following line. In categories of meaning, such as colour-words matched by colour-words, preposition by preposition, etc. This verbal parallelism sometimes extends to the first and, more rarely, to the last couplet as well.
Regulated verse and truncated are two basic forms of Jintishi, or Modern style poetry. Regulated verse poems has two forms, heptasyllabic and pentasyllabic: