1. each other
2. one another
3. mutually
4. fret on the neck of a pipa 琵琶 pípa (a fret on the soundboard is called a 品 pǐn)

1. letter
2. mail
3. to trust
4. to believe
5. to profess faith in
6. truthful
7. confidence
8. trust
9. at will
10. at random

classifier: 封