The key concept of the collective functions is to have a group or groups of participating processes. The routines do not have group identifiers as explicit arguments. Instead, there is a communicator argument. Groups and communicators are discussed in full detail in Chapter Groups, Contexts, Communicators, and Caching . For the purposes of this chapter, it is sufficient to know that there are two types of communicators: intra-communicators and inter-communicators. An intracommunicator can be thought of as an indentifier for a single group of processes linked with a context. An intercommunicator identifies two distinct groups of processes linked with a context.