Some varieties of English cannot be understood by speakers of different varieties of English because of differing syntax and morphology.

Some varieties of Chinese cannot be understood by speakers of different varieties of Chinese because of differing phonology.

Sometimes a speaker of language A can understand a variety of language B, but not understand a different variety of language A.

Sometimes a speaker of language A can understand language B, but a speaker of language B cannot understand language A

Some varieties of Chinese cannot be understood by speakers of different varieties of Chinese because of differing orthography.