  This paper introduces the notion of `commonly knowing whether', a
non-standard version of standard common knowledge which is defined on the basis
of `knowing whether', instead of standard `knowing that'. After giving five
possible definitions of this notion, we explore the logical relations among
them in the single-agent and multi-agent cases. We propose a sound and complete
axiomatization. We investigate one of the five definitions in terms of
expressivity via a strategy of modal comparison games.
