sent in messages through (other) channels, allowing the topology of process interconnections to change. Some process calculi also allow channels to be created during the execution of a computation. Communication Interaction can be (but isn't always) a directed flow of information. That is, input and output can be distinguished as dual interaction primitives. Process calculi that make such distinctions typically define an input operator (e.g. ) and an output operator (e.g. ), both of which name an interaction point (here ) that is used to synchronise with a dual interaction primitive. Should information be exchanged, it will flow from the outputting to the inputting process. The output primitive will specify the data to be sent. In , this data is . Similarly, if an input expects to receive data, one or more bound variables will act as place-holders to be substituted by data, when it arrives. In , plays that role. The choice of the kind of data that can be exchanged in an interaction is one of the key features that distinguishes different process calculi. Sequential composition Sometimes interactions must be temporally ordered. For example, it might be desirable to specify algorithms such as: first receive some data on and then send that data on . Sequential composition can be used for such purposes. It is well known from other models of computation. In process calculi, the sequentialisation operator is usually integrated with input or output, or both. For example, the process will wait for an input on . Only when this input has occurred will the process be activated, with the received data through substituted for identifier . Reduction semantics The key operational reduction rule, containing the computational essence of process calculi, can be given solely in terms of parallel composition, sequentialization, input, and output. The details of this reduction vary among the calculi, but the essence remains roughly the same. The reduction rule is: The interpretation to this reduction rule is: The process sends a message, here , along the channel . Dually, the process receives that message on channel . Once the message has been sent, becomes the process , while becomes the process , which is with the place-holder substituted by , the data received on . The class of processes that is allowed to range over as the continuation of the output operation substantially influences the properties of the calculus. Hiding Processes do not