What is the difference between [ ] and [[ ]] in R? [duplicate]

Question

  > levels(state.region)[2]
    [1] "S"
    > levels(state.region)[[2]]
    [1] "S"

They return the same value, so I don't know what is the difference between them.

Answer 1

[] = always returns object of same class (out of basic object classes), can select more than one element of an object

[[]] = can extract one element from list or data frame, returned object (out of basic object classes) not necessarily list/dataframe