Every word of metrical poetry (modern poetry) pays attention to flatness, but the words in some positions are flexible and even, so the analyst circles them. For example, the words on the picture provided by the subject are like this. Sometimes, this symbol circled outside the word, for example, "five-character flat", is the flat in the seven-character poem. Sometimes the division is even finer. The mark "⊙" should be flat and the mark "◎" should be flat. Both ⊙ and ◎ can be represented by "zhong".
About drawing a triangle under the word CuO, this is the symbol of CuO, but it is not static. Sometimes, the word "Yi" is represented by a black circle. For example, in The Book of Songs, Liu Yuxi's word "Yi Naqu":
The willows are green, and the bamboo branches have no complaints. ⊙●●○△ ●○○●△
As soon as Tong Lang turned around, he listened to the song. ⊙◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎◎967
As for drawing a circle under the flat-voiced word or using zero to represent the flat-voiced word, it is generally the same.