When formalising the rules of trust in the remote attesta- tion of TPM-based computer systems it is paramount that the rules are precisely understood, supporting unambiguous communication of infor- mation about system requirements between engineers. We present a dia- grammatic approach to modelling rules of trust using an extended version of concept diagrams. Within the context of our proof-of-concept Net- work Function Virtualisation and Attestation environment, these rules allow different level of trust to be explored and, importantly, allow us to identify when a computer system should not be trusted. To ensure that the modelling approach can be applied to general systems, we in- clude generic patterns for extending our domain model and rules of trust. Consequently, through the use of a formal, yet accessible, diagrammatic notation, domain experts can define rules of trust for their systems.