So there are three main geographical subdivisions.

有三种主要的地理形态分类。

耶鲁公开课 - 旧约导论课程节选

So let's think about how we can classify single and double and triple bonds, which is what we're really used to dealing with in terms of these sigma bonds and these pi bonds.

让我们想想怎么来分类单键双键,和三键，这在我们在考虑sigma键,和π键是很有用的。

麻省理工公开课 - 化学原理课程节选