单词nonrestrictive clause
网络: 非限制性从句；非限制性动词定语从句；非限制性定语从句
When a nonrestrictive clause appears at the end of a sentence, place a comma before it and a period after it.
当非限制*成分出现在句末时，在它前面加逗号，后面加句号。
When a nonrestrictive clause appears in the middle of a sentence, place commas around it.
当非限制*成分在句子中间出现时，应当在它前面和后面加逗号。
Use two commas, not one, to set off a nonrestrictive clause in the middle of a sentence.
用两个逗号，而不是一个，来分隔非限制性从句。