非限制性定语从句在句中主要起补充说明作用，在非限定性定语从句与主句之间往往有逗号隔开，独立存在，除掉也不会影响整个句子的意思。
1. as引导的非限制性定语从句
as有时作关系代词，代替整个主句，对其进行说明
但as也回用于固定搭配，像as we all know, as it is known, as is known to all, as it is, as is said above, as always mentioned above, as is usual, as is often the case, as is reported in the newspaper等。
as在非限定性定语从句中作主语、表语或宾语，且引出的从句位置比较灵活，可位于句首或句末，也可置于主句中间。
eg. As we all know, Jack London is a famous American writer.
2. which引导的非限制性定语从句
① which指代主句中的名词。
eg.These apple，which were bought by my father，are very sweet.
② which指代主句中的形容词。
eg.She was very patient towards the children，which her husband seldom was.
③ which指代主句中的某个从句。
eg.He said that he will love her forever，which was not true.
