A file synchronizer is a tool that reconciles disconnected modifications to a replicated directory structure. Trustworthy synchronizers are difficult to build, since they must deal correctly with both the semantic complexities of file systems and the unpredictable failure modes arising from distributed operation. On the other hand, synchronizers are often packaged as stand-alone, user-level utilities, whose intended behavior is relatively easy to isolate from the other functions of the system. This combination of subtlety and isolability makes file synchronizers attractive candidates for precise mathematical specification. We present here a detailed specification of a particular file synchronizer called Unison, sketch an idealized reference implementation of our specification, and discuss the relation between our idealized implementation and the actual code base.