Refers to two sentences. There are eight lines in the rhythmic poem, and every two lines form a couplet, which are called the first couplet, chin couplet, neck couplet and tail couplet in order.
For example, Du Fu's "Walking at Night"
On the shore, there is a gentle breeze with fine grass, and a lonely boat with dangerous rafts. (First couplet)
The stars hang over the vast plains, and the moon surges across the river. (Joint couplet)
It is not famous to write articles. Officials should retire due to old age and illness. (neck couplet)
What does Piao Piao look like? The sky and the earth are like sand gulls.
(last couplet)