  The cost of communication is a substantial factor affecting the scalability
of many distributed applications. Every message sent can incur a cost in
storage, computation, energy and bandwidth. Consequently, reducing the
communication costs of distributed applications is highly desirable. The best
way to reduce message costs is by communicating without sending any messages
whatsoever. This paper initiates a rigorous investigation into the use of
silence in synchronous settings, in which processes can fail. We formalize
sufficient conditions for information transfer using silence, as well as
necessary conditions for particular cases of interest. This allows us to
identify message patterns that enable communication through silence. In
particular, a pattern called a {\em silent choir} is identified, and shown to
be central to information transfer via silence in failure-prone systems. The
power of the new framework is demonstrated on the {\em atomic commitment}
problem (AC). A complete characterization of the tradeoff between message
complexity and round complexity in the synchronous model with crash failures is
provided, in terms of lower bounds and matching protocols. In particular, a new
message-optimal AC protocol is designed using silence, in which processes
decide in~3 rounds in the common case. This significantly improves on the best
previously known message-optimal AC protocol, in which decisions were performed
in $\Theta(n)$ rounds.
