A pass-through Actor is usually a device that just passes along input and outputs from other Actors. For example: consider a computer keyboard and the monitor; they act as intermediaries between the end user and the computer system. Unless you are designing the hardware, these devices are transparent, and they do not need to be Actors. Instead, consider the end user as the Actor and consider the pass-through Actors as internal devices (or design elements) of the system under consideration.