國 破 山 河 在 城 春 草 木 深
In Classical Chinese poetry, the presence or absence of formal tonal constraints regarding tone pattern varies according to the poetic form of a specific poem.
Sometimes the rules governing the permissible tone patterns for a poem were quite strict, yet still allowed for a certain amount of liberty and variation, as in the case of regulated verse.
In the fixed-tone pattern type of verse, poems were written according to preexisting models known as "tunes".
This was the case with the ci and the qu: an individual poem was written so that its tone pattern (and line lengths) were the same as one of the model types, the poetic variation was in the change in the particular wording of the lyrics.