Because of the tendency to write poetry as groups of couplets, most poems had an even number of lines. Generally four lines (two couplets) were considered to be the minimum length for a poem. In the case of curtailed verse (jueju), the poem was limited to this length. Other types of poems were limited to eight lines (four couplets). If the overall length of some form of poetry was not limited, the poems tended to be written using four- or eight-line stanzas, so the poem lengths would work out to multiples of four or eight. Some poems were quite long. The length of poems based upon specific song and ballad forms depended upon the specific tune or form selected as the model. The fu type of poem, which sometimes even incorporated sections of prose, had few limitations on line length, except that, within a section of verse, the line lengths tended to be of equal length.Frankel, 212–213 A specific poem's length for those forms in which this was a restriction, is another basic classifying criterion (as in Seven-character eight-line verse).
