  We (re)define session types as projections of process behaviors with respect
to the communication channels they use. In this setting, we give session types
a semantics based on fair testing. The outcome is a unified theory of
behavioral types that shares common aspects with conversation types and that
encompass features of both dyadic and multi-party session types. The point of
view we provide sheds light on the nature of session types and gives us a
chance to reason about them in a framework where every notion, from
well-typedness to the subtyping relation between session types, is semantically
-rather than syntactically- grounded.
