Near the end of the [ch19-04-advanced-types.md](https://github.com/rust-lang/book/blob/master/src/ch19-04-advanced-types.md): > A trait bound on ?Sized is the opposite of a trait bound on Sized: we would read this as “T may or may not be Sized.” This is not describing the opposite, more like superset relation. following immediately: > This syntax is only available for Sized, not any other traits. It just described that it "may or may not be Sized" but then "only available for Sized" ...?