This is a study on the function of the prototyping process in game design. It is based on interviews with 27 game designers in leading positions at companies of various sizes. Prototyping is an important part of game design with which design ideas are explored. One central purpose of prototypes is to serve as a communicational tool. As such it is used to negotiate design problems. Rhetoric has a long tradition of analyzing communication and negotiation. In this paper a number of concepts from rhetoric, (topos, hodos, pistis, partes and to some extent synecdoche) are applied to game prototyping based on data collected as interviews. The results indicate that rhetoric concepts are useful when talking about the prototypes as they grasp the qualities of a prototyping in a good way. By applying the findings using negotiation theory to real practice the game prototyping process would likely become clearer without diminishing its creative qualities. As presented here negotiation theory could serve as a conceptual framework for game prototyping, which the design team can make use of in their design process.