大方廣圓覺修多羅了義經略疏 2.606

2.606

五、品字單雙觀。上單靜觀如上一口，後雙明寂幻如下兩口，故云單雙。

Fifth, the contemplation of the pin-character single and double. The single stillness contemplation above is like the single top "mouth" [component of the character 品], and subsequently clarifying quiescence and illusion doubly is like the bottom two "mouths"; hence it is called "single and double."